Inside of the interactive ruby console if i type ri then i get an undefined method error, do i explicitly have to install documentation somewhere to get this to work?

irb(main):015:0* ri --help
NoMethodError: undefined method `-@' for nil:NilClass
    from (irb):15
    from /usr/local/lib/ruby/site_ruby/1.8/rubygems.rb:295
irb(main):016:0> ri Array
NoMethodError: undefined method `ri' for main:Object
    from (irb):16
    from /usr/local/lib/ruby/site_ruby/1.8/rubygems.rb:295