(erb):52:in block (2 levels) in get_binding': undefined method split' for nil:NilClass (NoMethodError)

[ain]

Task fails with the following error:

Custom markdown renderer Cortanamarkdownrenderer loaded.
Adding renderer for haml examples
Adding renderer for html examples
Adding renderer for js examples
Adding renderer for jsx examples
Adding renderer for react examples
Adding renderer for slim examples
(erb):52:in `block (2 levels) in get_binding': undefined method `split' for nil:NilClass (NoMethodError)
        from (erb):49:in `each'
        from (erb):49:in `block in get_binding'
        from (erb):46:in `each'
        from (erb):46:in `get_binding'
        from /Users/ain/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/erb.rb:850:in `eval'
        from /Users/ain/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/erb.rb:850:in `result'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:243:in `write_page'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:199:in `block in write_docs'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:185:in `each'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:185:in `write_docs'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:147:in `build_docs'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:87:in `build'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/cli.rb:38:in `build'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/lib/hologram/cli.rb:30:in `run'
        from /Users/ain/.rvm/gems/ruby-2.1.5@global/gems/hologram-1.4.0/bin/hologram:6:in `<top (required)>'
        from /Users/ain/.rvm/rubies/ruby-2.1.5/bin/hologram:23:in `load'
        from /Users/ain/.rvm/rubies/ruby-2.1.5/bin/hologram:23:in `<main>'
        from /Users/ain/.rvm/gems/ruby-2.1.5/bin/ruby_executable_hooks:15:in `eval'
        from /Users/ain/.rvm/gems/ruby-2.1.5/bin/ruby_executable_hooks:15:in `<main>'

Platform:

OS X with RVM
Ruby 2.2.2 (or 2.1.5)
hologram 1.4.0
grunt-cli v0.1.13
grunt v0.4.5