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When I add this gem into my app Gemfile : gem 'mongoid', '~> 2.3.2. 
I'm unable to run any "rails g" command, it cause this stack trace : 

/MY_PATH/vendor/bundle/ruby/1.9.1/gems/activesupport-3.1.1/lib/active_support/whiny_nil.rb:48:in `method_missing': undefined method `values' for nil:NilClass (NoMethodError)
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/mongoid-2.3.2/lib/mongoid/railtie.rb:64:in `block in <class:Railtie>'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/initializable.rb:30:in `instance_exec'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/initializable.rb:30:in `run'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/initializable.rb:55:in `block in run_initializers'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/initializable.rb:54:in `each'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/initializable.rb:54:in `run_initializers'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/application.rb:96:in `initialize!'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/railtie/configurable.rb:30:in `method_missing'
	from /server/poker/config/environment.rb:5:in `<top (required)>'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/application.rb:83:in `require'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/application.rb:83:in `require_environment!'
	from /server/poker/vendor/bundle/ruby/1.9.1/gems/railties-3.1.1/lib/rails/commands.rb:22:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'