Not working when used with jekyll-category-archive-plugin

[eberkund]

When I use either plugin on its own the site works fine but when I use both together I get this error:

jekyll serve --trace /Users/erik/.rvm/gems/ruby-head/gems/liquid-2.6.1/lib/liquid/htmltags.rb:43: warning: duplicated key at line 46 ignored: "index0" Configuration file: /Users/erik/chessground/_config.yml Source: /Users/erik/chessground Destination: /Users/erik/chessground/_site Generating... /Users/erik/chessground/_plugins/generate_sitemap.rb:84:in '+': no implicit conversion of nil into String (TypeError) from /Users/erik/chessground/_plugins/generate_sitemap.rb:84:in 'block in generate_content' from /Users/erik/chessground/_plugins/generate_sitemap.rb:83:in 'each' from /Users/erik/chessground/_plugins/generate_sitemap.rb:83:in 'generate_content' from /Users/erik/chessground/_plugins/generate_sitemap.rb:60:in 'block in generate' from /Users/erik/chessground/_plugins/generate_sitemap.rb:58:in 'open' from /Users/erik/chessground/_plugins/generate_sitemap.rb:58:in 'generate' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/site.rb:280:in 'block in generate' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/site.rb:279:in 'each' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/site.rb:279:in 'generate' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/site.rb:50:in 'process' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/command.rb:28:in 'process_site' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/commands/build.rb:56:in 'build' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/commands/build.rb:34:in 'process' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/lib/jekyll/commands/serve.rb:26:in 'block (2 levels) in init_with_program' from /Users/erik/.rvm/gems/ruby-head/gems/mercenary-0.3.5/lib/mercenary/command.rb:220:in 'call' from /Users/erik/.rvm/gems/ruby-head/gems/mercenary-0.3.5/lib/mercenary/command.rb:220:in 'block in execute' from /Users/erik/.rvm/gems/ruby-head/gems/mercenary-0.3.5/lib/mercenary/command.rb:220:in 'each' from /Users/erik/.rvm/gems/ruby-head/gems/mercenary-0.3.5/lib/mercenary/command.rb:220:in 'execute' from /Users/erik/.rvm/gems/ruby-head/gems/mercenary-0.3.5/lib/mercenary/program.rb:42:in 'go' from /Users/erik/.rvm/gems/ruby-head/gems/mercenary-0.3.5/lib/mercenary.rb:19:in 'program' from /Users/erik/.rvm/gems/ruby-head/gems/jekyll-2.5.2/bin/jekyll:20:in '<top (required)>' from /Users/erik/.rvm/gems/ruby-head/bin/jekyll:23:in 'load' from /Users/erik/.rvm/gems/ruby-head/bin/jekyll:23:in '<main>' from /Users/erik/.rvm/gems/ruby-head/bin/ruby_executable_hooks:15:in 'eval' from /Users/erik/.rvm/gems/ruby-head/bin/ruby_executable_hooks:15:in '<main>'