title: jekyll 3.0.1 | Error: undefined method `map!' for nil:NilClass
body: <p>I wanted to do an update to a jekyll site - but when trying out "jekyll serve" I now get:</p>
  
  <pre><code>WARN: Unresolved specs during Gem::Specification.reset:
        jekyll-watch (~&gt; 1.1)
  WARN: Clearing out unresolved specs.
  Please report a bug if this causes problems.
  Configuration file: /home/ligi/git/espass.github.io/_config.yml
  jekyll 3.0.1 | Error:  undefined method `map!' for nil:NilClass
  </code></pre>
  
  <p>also fails with bundle exec jekyll serve </p>
  
  <pre><code>➜  espass.github.io git:(master) ✗ bundle exec jekyll serve 
  Configuration file: /home/ligi/git/espass.github.io/_config.yml
  jekyll 3.0.1 | Error:  undefined method `map!' for nil:NilClass
  </code></pre>
  
  <p>Anyone knows how to trace down this error? Don't find anything about it in the internet and am kinda stuck here</p>
  
  <p>Edit:</p>
  
  <pre><code>➜  espass.github.io git:(master) ✗ bundle exec jekyll serve --trace
  Configuration file: /home/ligi/git/espass.github.io/_config.yml
  /usr/lib/ruby/vendor_ruby/jekyll/configuration.rb:246:in `block in backwards_compatibilize': undefined method `map!' for nil:NilClass (NoMethodError)
      from /usr/lib/ruby/vendor_ruby/jekyll/configuration.rb:238:in `each'
      from /usr/lib/ruby/vendor_ruby/jekyll/configuration.rb:238:in `backwards_compatibilize'
      from /usr/lib/ruby/vendor_ruby/jekyll/configuration.rb:189:in `read_config_files'
      from /var/lib/gems/2.3.0/gems/jekyll-3.0.5/lib/jekyll.rb:101:in `configuration'
      from /usr/lib/ruby/vendor_ruby/jekyll/command.rb:42:in `configuration_from_options'
      from /var/lib/gems/2.3.0/gems/jekyll-3.0.5/lib/jekyll/commands/build.rb:29:in `process'
      from /var/lib/gems/2.3.0/gems/jekyll-3.0.5/lib/jekyll/commands/serve.rb:26:in `block (2 levels) in init_with_program'
      from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `block in execute'
      from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `each'
      from /usr/lib/ruby/vendor_ruby/mercenary/command.rb:220:in `execute'
      from /usr/lib/ruby/vendor_ruby/mercenary/program.rb:42:in `go'
      from /usr/lib/ruby/vendor_ruby/mercenary.rb:19:in `program'
      from /var/lib/gems/2.3.0/gems/jekyll-3.0.5/bin/jekyll:17:in `&lt;top (required)&gt;'
      from /usr/local/bin/jekyll:23:in `load'
      from /usr/local/bin/jekyll:23:in `&lt;main&gt;'
  </code></pre>