"no implicit conversion of nil into String (TypeError)" with Jekyll 2.0.2

[yizeng]

Seen this error while using jekyll serve --trace with latest Jekyll gem, where I had no issue with Jekyll gem 1.5.1.
It could be somehow related to some static files in my repository or some configurations I need to update, as jekyllrb.com can be built successfully on my machine.
Environment:

• Windows 7, Ruby 2.0.0p451, Jekyll gem 2.0.2
• Linux Mint 16, Ruby 2.1.1p76, Jekyll gem 2.0.2

Stacktrace (static_file.rb#L21):

D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/static_file.rb:21:in `join': no implicit conversion of nil into String (TypeError)
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/static_file.rb:21:in `path'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/static_file.rb:26:in `relative_path'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/site.rb:323:in `block in site_payload'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/site.rb:323:in `sort'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/site.rb:323:in `site_payload'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/site.rb:243:in `render'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/site.rb:43:in `process'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/command.rb:43:in `process_site'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/commands/build.rb:46:in `build'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/commands/build.rb:30:in `process'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/lib/jekyll/commands/serve.rb:23:in `block (2 levels) i
    n init_with_program'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/mercenary-0.3.2/lib/mercenary/command.rb:220:in `call'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/mercenary-0.3.2/lib/mercenary/command.rb:220:in `block in execute'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/mercenary-0.3.2/lib/mercenary/command.rb:220:in `each'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/mercenary-0.3.2/lib/mercenary/command.rb:220:in `execute'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/mercenary-0.3.2/lib/mercenary/program.rb:35:in `go'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/mercenary-0.3.2/lib/mercenary.rb:22:in `program'
    from D:/Ruby200-x64/lib/ruby/gems/2.0.0/gems/jekyll-2.0.2/bin/jekyll:18:in `<top (required)>'
    from D:/Ruby200-x64/bin/jekyll:23:in `load'
    from D:/Ruby200-x64/bin/jekyll:23:in `<main>'