execption notifier gem issue

execption notifier gem issue

I installed exception_notifier gem following this
http://railscasts.com/episodes/104-exception-notifications-revised. But
when running rails s, I got this
/home/ruby-2.0.0-p195/gems/actionpack-3.2.13/lib/action_dispatch/middleware/stack.rb:43:in
`build': undefined method `new' for ExceptionNotifier:Module
(NoMethodError)
So I tried to Google this problem, and I found that I could use in my
production.rb file this config.middleware.use ExceptionNotifier::Rack...
insted of this config.middleware.use ExceptionNotifier...
But then I got this message:
uninitialized constant ExceptionNotifier::Rack (NameError)
How should I handle this gem installation ?