OSX Mojave : Error: undefined method `undent' for #<String:0x00007fe1c6925180>

I'm running OSX 10.14.4 (Mojave) and have run into this error when trying to install credulous

brew is up to date : Homebrew/homebrew-core (git revision 2a40; last commit 2019-04-26)

The error

>> brew install https://raw.githubusercontent.com/realestate-com-au/credulous-brew/master/credulous.rb
######################################################################## 100.0%
==> Downloading https://github.com/realestate-com-au/credulous/releases/download/0.2.1/credulous-0.2.1.131-osx.tgz
==> Downloading from https://github-production-release-asset-2e65be.s3.amazonaws.com/19685500/ebb13d16-ecab-11e3-84c3-d13c0253e593?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYA
######################################################################## 100.0%
Error: undefined method `undent' for #<String:0x00007fe1c6925180>
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/Users/mohamed/Library/Caches/Homebrew/Formula/credulous.rb:26:in `caveats'
/usr/local/Homebrew/Library/Homebrew/caveats.rb:17:in `caveats'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/forwardable.rb:202:in `empty?'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:609:in `caveats'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:658:in `finish'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:331:in `install_formula'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:262:in `block in install'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:260:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:260:in `install'
/usr/local/Homebrew/Library/Homebrew/brew.rb:100:in `<main>'

Any idea how to fix ?

realestate-com-au / credulous

OSX Mojave : Error: undefined method `undent' for #<String:0x00007fe1c6925180> #91