title: Uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)
body: <p>When I want to create a Ruby on Rails project, I get the message below.</p>
  
  <pre><code>/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.8/lib/active_support/dependencies.rb:55: uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)
      from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
      from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `require'
      from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.8/lib/active_support.rb:57
      from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
      from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `require'
      from /usr/lib/ruby/gems/1.8/gems/rails-2.3.8/lib/rails_generator.rb:31
      from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
      from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in `require'
      from /usr/lib/ruby/gems/1.8/gems/rails-2.3.8/bin/rails:15
      from /usr/bin/rails:19:in `load'
      from /usr/bin/rails:19
  </code></pre>
  
  <p>What has gone wrong? How do I to fix it?</p>