rake doc fails, even after installing pandoc

[indirect]

Repro steps:

1. brew install pandoc
2. hub clone bundler/gemstash
3. bundle install
4. bundle exec rake doc

pandoc: Error running filter /Users/andre/src/bundler/gemstash/rake/doc/insert_github_generation_comment.rb
Filter returned error status 1
groff -Wall -mtty-char -mandoc -Tascii /Users/andre/src/bundler/gemstash/lib/gemstash/man/gemstash-readme.7 | col -b > /Users/andre/src/bundler/gemstash/lib/gemstash/man/gemstash-readme.7.txt
troff: fatal error: can't open `/Users/andre/src/bundler/gemstash/lib/gemstash/man/gemstash-readme.7': No such file or directory
pandoc -s -f markdown -t html --filter '/Users/andre/src/bundler/gemstash/rake/doc/insert_github_generation_comment.rb' --filter '/Users/andre/src/bundler/gemstash/rake/doc/insert_images.rb' --filter '/Users/andre/src/bundler/gemstash/rake/doc/upcase_headers.rb' --filter '/Users/andre/src/bundler/gemstash/rake/doc/fix_links_for_format.rb' -o '/Users/andre/src/bundler/gemstash/html/index.html' '/Users/andre/src/bundler/gemstash/man/gemstash-readme.7.md'
/Users/andre/src/bundler/gemstash/.bundle/ruby/2.3.0/gems/pandoc_object_filters-0.1.0/lib/pandoc_object_filters/element/document.rb:15:in `to_ast': undefined method `to_ast' for nil:NilClass (NoMethodError)
Did you mean?  to_a
               to_s
        from /Users/andre/src/bundler/gemstash/.bundle/ruby/2.3.0/gems/pandoc_object_filters-0.1.0/lib/pandoc_object_filters/element.rb:18:in `to_ast'
        from /Users/andre/src/bundler/gemstash/.bundle/ruby/2.3.0/gems/pandoc_object_filters-0.1.0/lib/pandoc_object_filters/filter.rb:34:in `process'
        from /Users/andre/src/bundler/gemstash/.bundle/ruby/2.3.0/gems/pandoc_object_filters-0.1.0/lib/pandoc_object_filters/filter.rb:23:in `filter!'
        from /Users/andre/src/bundler/gemstash/rake/doc/insert_github_generation_comment.rb:34:in `<main>'

Is this a bug in pandoc_object_filters? /cc @smellsblue