confused about cache dir?

I'm new to fpm-cookery. I've installed today (fpm-cookery v0.31.0 (fpm v1.4.0)) and try to learn from your recipes collection, but can't explain following issue

$ fpm-cook
===> Starting package creation for foo-0.1.0 (ubuntu, deb)
===> 
===> Verifying build_depends and depends with Puppet
===> All build_depends and depends packages installed
===> Fetching source: 
===> Using cached file /root/test/cache
/var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/source_integrity_check.rb:63:in `read': Is a directory - /root/test/cache (Errno::EISDIR)
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/source_integrity_check.rb:63:in `block in build_checksum'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/source_integrity_check.rb:62:in `open'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/source_integrity_check.rb:62:in `build_checksum'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/source_integrity_check.rb:37:in `verify!'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/source_integrity_check.rb:15:in `initialize'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/packager.rb:90:in `new'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/packager.rb:90:in `block in dispense'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/packager.rb:83:in `chdir'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/packager.rb:83:in `dispense'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/cli.rb:113:in `exec'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/cli.rb:77:in `block in execute'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/book.rb:16:in `call'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/book.rb:16:in `load_recipe'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/lib/fpm/cookery/cli.rb:73:in `execute'
    from /var/lib/gems/1.9.1/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'
    from /var/lib/gems/1.9.1/gems/clamp-0.6.5/lib/clamp/subcommand/execution.rb:11:in `execute'
    from /var/lib/gems/1.9.1/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'
    from /var/lib/gems/1.9.1/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'
    from /var/lib/gems/1.9.1/gems/fpm-cookery-0.31.0/bin/fpm-cook:7:in `<top (required)>'
    from /usr/local/bin/fpm-cook:23:in `load'
    from /usr/local/bin/fpm-cook:23:in `<main>'

This is my recipe:

class Foo < FPM::Cookery::Recipe
  description 'foo'

  name     'foo'
  version  '0.1.0'
  revision '1'

  arch     'all'
  section  'main'

 # depends        'openssh-client (>=1.6.6)'

  def build
    # nothing to do here
  end

  def install
    usr('local').install workdir('README.md')
  end
end

I've just want to package one file (for testing purpose). The mentioned README.md is located in same directory as the recipe.rb (/root/test)

What's my mistake here?

bernd / fpm-cookery

confused about cache dir? #141