`brew doctor` fails with the error: `Error: no implicit conversion of nil into String`

[jemartti]

Please note we will close your issue without comment if you delete, do not read or do not fill out the issue checklist below and provide ALL the requested information. If you repeatedly fail to use the issue template, we will block you from ever submitting issues to Homebrew again.

• [x] are reporting a bug others will be able to reproduce and not asking a question. If you're not sure or want to ask a question do so on our Discourse: https://discourse.brew.sh
• [ ] ran a brew command and reproduced the problem with multiple formulae? If it's a problem with a single, official formula (not cask) please file this issue at Homebrew/homebrew-core: https://github.com/Homebrew/homebrew-core/issues/new/choose. If it's a brew cask problem please file this issue at https://github.com/Homebrew/homebrew-cask/issues/new/choose. If it's a tap (e.g. Homebrew/homebrew-php) problem please file this issue at the tap.
• [x] ran brew update and can still reproduce the problem?
• [x] ran brew doctor, fixed all issues and can still reproduce the problem?
• [x] ran brew config and brew doctor and included their output with your issue?

To help us debug your issue please explain:

• What you were trying to do (and why)

After running brew update this morning, brew doctor consistently fails with the error Error: no implicit conversion of nil into String.

• What happened (include command output)

-> brew doctor
Error: no implicit conversion of nil into String
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `extname'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `loader_for'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:280:in `factory'
/usr/local/Homebrew/Library/Homebrew/dependency.rb:33:in `to_formula'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `map'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `missing_dependencies'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:13:in `block in missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `each'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:720:in `check_missing_deps'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:46:in `block in doctor'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `doctor'
/usr/local/Homebrew/Library/Homebrew/brew.rb:95:in `<main>'

• What you expected to happen

I expected brew doctor to return either a clean bill of health or a specific set of problems.

• Step-by-step reproduction instructions (by running brew commands)

-> brew update
Already up-to-date.
-> brew doctor
Error: no implicit conversion of nil into String
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `extname'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `loader_for'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:280:in `factory'
/usr/local/Homebrew/Library/Homebrew/dependency.rb:33:in `to_formula'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `map'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `missing_dependencies'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:13:in `block in missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `each'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:720:in `check_missing_deps'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:46:in `block in doctor'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `doctor'
/usr/local/Homebrew/Library/Homebrew/brew.rb:95:in `<main>'

Output of brew config:

-> brew config
HOMEBREW_VERSION: 1.6.6-63-ga674a97
ORIGIN: https://github.com/Homebrew/brew.git
HEAD: a674a972a56850a6f76eb1d9e812438bc435152b
Last commit: 65 minutes ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 44ca0d278857397e87273740bb3229772cd733e5
Core tap last commit: 2 hours ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_AWS_ACCESS_KEY_ID: set
HOMEBREW_AWS_SECRET_ACCESS_KEY: set
HOMEBREW_DEV_CMD_RUN: 1
CPU: octa-core 64-bit haswell
Homebrew Ruby: 2.3.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Clang: 9.1 build 902
Git: 2.17.1 => /usr/local/bin/git
Curl: 7.54.0 => /usr/bin/curl
Java: 1.8.0_72
macOS: 10.13.4-x86_64
CLT: 9.4.0.0.1.1526532315
Xcode: 9.4
XQuartz: 2.7.11 => /opt/X11

[scpeters]

see also #4262

[scpeters]

I think this error is coming from a specific formula that you have installed. Can you try brew doctor --debug? That might show which formula it is.

[MikeMcQuaid]

brew doctor --debug --verbose would be even better. I can't reproduce this as-is but am trying to do so.

[jemartti]

-> brew doctor --debug --verbose
Checking check_access_cache
Checking check_access_cellar
Checking check_access_homebrew_repository
Checking check_access_lock_dir
Checking check_access_logs
Checking check_access_prefix_directories
Checking check_access_site_packages
Checking check_brew_git_origin
Checking check_build_from_source
Checking check_clt_minimum_version
Checking check_clt_up_to_date
Checking check_coretap_git_origin
Checking check_filesystem_case_sensitive
Checking check_for_anaconda
Checking check_for_bad_curl
Checking check_for_bad_python_symlink
Checking check_for_beta_xquartz
Checking check_for_config_scripts
Checking check_for_external_cmd_name_conflict
Checking check_for_gettext
Checking check_for_git
Checking check_for_iconv
Checking check_for_installed_developer_tools
Checking check_for_multiple_volumes
Checking check_for_non_prefixed_coreutils
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FormulaLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/coreutils.rb
Checking check_for_non_prefixed_findutils
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FormulaLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/findutils.rb
Checking check_for_other_frameworks
Checking check_for_other_package_managers
Checking check_for_pydistutils_cfg_in_home
Checking check_for_stray_dylibs
Checking check_for_stray_headers
Checking check_for_stray_las
Checking check_for_stray_pcs
Checking check_for_stray_static_libs
Checking check_for_symlinked_cellar
Checking check_for_tap_ruby_files_locations
Checking check_for_unlinked_but_not_keg_only
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/ncurses.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/expat.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/apr-util.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/icu4c.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/readline.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/nss.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/sqlite.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/curl.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gettext.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/apr.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/openssl.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/bison.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libffi.rb
Checking check_for_unsupported_macos
Checking check_git_newline_settings
Checking check_git_status
Checking check_git_version
Checking check_homebrew_prefix
Checking check_if_xcode_needs_clt_installed
Checking check_ld_vars
Checking check_multiple_cellars
Checking check_ruby_version
Checking check_tmpdir
Checking check_tmpdir_sticky_bit
Checking check_user_path_1
Checking check_user_path_2
Checking check_user_path_3
Checking check_xcode_license_approved
Checking check_xcode_minimum_version
Checking check_xcode_prefix
Checking check_xcode_prefix_exists
Checking check_xcode_select_path
Checking check_xcode_up_to_date
Checking check_xquartz_up_to_date
Checking check_for_broken_symlinks
Checking check_missing_deps
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/mosh.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/tig.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/lame.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pkg-config.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libtiff.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libtool.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gmp.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/ffmpeg.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/fdk-aac.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/autoenv.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/xvid.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/python@2.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/go.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libpng.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/nettle.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pixman.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gdbm.rb
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/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/freetype.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/glide.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/log4cxx.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/python.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/redis.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libvorbis.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/boost.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/kylef/homebrew-formulae/Formula/swiftenv.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/sox.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libunistring.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libogg.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/glib.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/graphicsmagick.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/fluid-synth.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/carthage.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/go-delve/homebrew-delve/Formula/delve.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/cairo.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/ruby-build.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gobject-introspection.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/xz.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/nvm.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/aws-elasticbeanstalk.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/fontconfig.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/mad.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libav.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libvpx.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/flac.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/keychain.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/mercurial.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/dep.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/autoconf.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libtasn1.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/oniguruma.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/youtube-dl.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/cockroach.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gstreamer.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pcre.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/x264.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/doxygen.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/faac.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/jpeg.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/nmap.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/jq.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libnice.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/nspr.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/srtp.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/imagemagick.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/makedepend.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/bash-completion.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libsndfile.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/jasper.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/p11-kit.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/cocoapods.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/protobuf.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/fswatch.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/rbenv.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/automake.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/git.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gnutls.rb
Error: no implicit conversion of nil into String
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `extname'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `loader_for'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:280:in `factory'
/usr/local/Homebrew/Library/Homebrew/dependency.rb:33:in `to_formula'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `map'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `missing_dependencies'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:13:in `block in missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `each'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:720:in `check_missing_deps'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:46:in `block in doctor'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `doctor'
/usr/local/Homebrew/Library/Homebrew/brew.rb:95:in `<main>'

[scpeters]

I can reproduce with gnutls or python@2 installed on my system

[rifaidev]

I have the same issue, below is the output of brew doctor --debug --verbose

Checking check_access_cache
Checking check_access_cellar
Checking check_access_homebrew_repository
Checking check_access_lock_dir
Checking check_access_logs
Checking check_access_prefix_directories
Checking check_access_site_packages
Checking check_brew_git_origin
Checking check_build_from_source
Checking check_clt_minimum_version
Checking check_clt_up_to_date
Checking check_coretap_git_origin
Checking check_filesystem_case_sensitive
Checking check_for_anaconda
Checking check_for_bad_curl
Checking check_for_bad_python_symlink
Checking check_for_beta_xquartz
Checking check_for_config_scripts
Checking check_for_external_cmd_name_conflict
Checking check_for_gettext
Checking check_for_git
Checking check_for_iconv
Checking check_for_installed_developer_tools
Checking check_for_multiple_volumes
Checking check_for_non_prefixed_coreutils
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FormulaLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/coreutils.rb
Checking check_for_non_prefixed_findutils
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::FormulaLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/findutils.rb
Checking check_for_other_frameworks
Checking check_for_other_package_managers
Checking check_for_pydistutils_cfg_in_home
Checking check_for_stray_dylibs
Checking check_for_stray_headers
Checking check_for_stray_las
Checking check_for_stray_pcs
Checking check_for_stray_static_libs
Checking check_for_symlinked_cellar
Checking check_for_tap_ruby_files_locations
Checking check_for_unlinked_but_not_keg_only
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/icu4c.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/readline.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/sqlite.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gettext.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/openssl.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libffi.rb
Checking check_for_unsupported_macos
Checking check_git_newline_settings
Checking check_git_status
Checking check_git_version
Checking check_homebrew_prefix
Checking check_if_xcode_needs_clt_installed
Checking check_ld_vars
Checking check_multiple_cellars
Checking check_ruby_version
Checking check_tmpdir
Checking check_tmpdir_sticky_bit
Checking check_user_path_1
Checking check_user_path_2
Checking check_user_path_3
Checking check_xcode_license_approved
Checking check_xcode_minimum_version
Checking check_xcode_prefix
Checking check_xcode_prefix_exists
Checking check_xcode_select_path
Checking check_xcode_up_to_date
Checking check_xquartz_up_to_date
Checking check_for_broken_symlinks
Checking check_missing_deps
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pkg-config.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/tree.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libtiff.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libtool.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gmp.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/wget.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pinentry.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libidn2.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/mas.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libksba.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/go.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/zsh-autosuggestions.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libpng.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/nettle.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gdbm.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/iproute2mac.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/freetype.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/python.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libunistring.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/composer.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libassuan.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libgpg-error.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/yarn.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/xz.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/vitorgalvao/homebrew-tiny-scripts/cask-repair.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libgcrypt.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/speedtest-cli.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/hr.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/firebase-cli.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libtasn1.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/node.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/jpeg.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/zsh.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/zsh-syntax-highlighting.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/libusb.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/prospectone/homebrew-perfops/perfops.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/imagemagick.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/guetzli.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/hub.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gnupg.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pinentry-mac.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/p11-kit.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/git.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/gnutls.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/dnsmasq.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/npth.rb
/usr/local/Homebrew/Library/Homebrew/brew.rb (Formulary::TapLoader): loading /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/adns.rb
Error: no implicit conversion of nil into String
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `extname'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:363:in `loader_for'
/usr/local/Homebrew/Library/Homebrew/formulary.rb:280:in `factory'
/usr/local/Homebrew/Library/Homebrew/dependency.rb:33:in `to_formula'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `map'
/usr/local/Homebrew/Library/Homebrew/formula.rb:1564:in `missing_dependencies'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:13:in `block in missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `each'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:12:in `missing_deps'
/usr/local/Homebrew/Library/Homebrew/diagnostic.rb:720:in `check_missing_deps'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:46:in `block in doctor'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/doctor.rb:38:in `doctor'
/usr/local/Homebrew/Library/Homebrew/brew.rb:95:in `<main>'

[scpeters]

Also can reproduce with brew missing instead of the full brew doctor