undefined method `downcase' for nil:NilClass (NoMethodError)

[arozwalak]

running hologram on my project results below error. I don't code in ruby and I can't find any solution for this issue. Maybe you know where is the problem?

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):37:in `block in get_binding': undefined method `downcase' for nil:NilClass (NoMethodError)
    from (erb):36:in `each'
    from (erb):36:in `get_binding'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/erb.rb:849:in `eval'
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/erb.rb:849:in `result'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:243:in `write_page'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:199:in `block in write_docs'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:185:in `each'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:185:in `write_docs'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:147:in `build_docs'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/doc_builder.rb:87:in `build'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/cli.rb:38:in `build'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/lib/hologram/cli.rb:30:in `run'
    from /Library/Ruby/Gems/2.0.0/gems/hologram-1.4.0/bin/hologram:6:in `<top (required)>'
    from /usr/bin/hologram:23:in `load'
    from /usr/bin/hologram:23:in `<main>'

[arozwalak]

I found a solutions. It was connected with hologram config.yml file and index option which was pointed to wrong category. Because of that _header.html couldn't read title variable on which it tried to fire downcase method.

Problem solved.