require 'mongo_mapper' causes exception in active_support

gistfile1.txt

[nwiger@pdsd65-31-27:mongo]$ irb
irb(main):001:0> require 'mongo_mapper'
NameError: uninitialized constant MongoMapper::Associations::ManyDocumentsProxy::MongoMapper
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:105:in `rescue in const_missing'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:94:in `const_missing'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/mongo_mapper-0.5.5/lib/mongo_mapper/associations/many_documents_proxy.rb:7:in `<class:ManyDocumentsProxy>'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/mongo_mapper-0.5.5/lib/mongo_mapper/associations/many_documents_proxy.rb:3:in `<module:Associations>'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/mongo_mapper-0.5.5/lib/mongo_mapper/associations/many_documents_proxy.rb:2:in `<module:MongoMapper>'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/mongo_mapper-0.5.5/lib/mongo_mapper/associations/many_documents_proxy.rb:1:in `<top (required)>'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:156:in `require'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:156:in `block in require'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:521:in `new_constants_in'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/activesupport-2.3.4/lib/active_support/dependencies.rb:156:in `require'
	from /usr/playerconnect-stack-2009e/lib/ruby/gems/1.9.1/gems/mongo_mapper-0.5.5/lib/mongo_mapper.rb:90:in `<top (required)>'
	from (irb):1:in `require'
	from (irb):1
	from /usr/playerconnect-stack-2009e/bin/irb:12:in `<main>'