error generating rails guides on rvm

Hi I've pulled down the latest code for rails from github. In my RVM environment I get the following error when trying to generate the rails guides.

Would anyone be able to tell me how to fix this?

Thanks!

rake generate_guides --trace
(in /home/yadav/dev/rails_edge/railties)
** Invoke generate_guides (first_time)
** Execute generate_guides
/home/yadav/.rvm/rubies/ruby-1.9.2-p290/bin/ruby guides/rails_guides.rb
Generating configuring.html
/home/yadav/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require': no such file to load -- multi_json (LoadError)
         from /home/yadav/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
         from /home/yadav/dev/rails_edge/activesupport/lib/active_support/dependencies.rb:236:in `block in require'
         from /home/yadav/dev/rails_edge/activesupport/lib/active_support/dependencies.rb:222:in `block in load_dependency'
         from /home/yadav/dev/rails_edge/activesupport/lib/active_support/dependencies.rb:616:in `new_constants_in'