"Undefined method merge" when using sweeper


I'm trying to use sweepers for the first time and got a problem - I
get "undefined method `merge' for "/signature/
f2d7c7c66450b169.html":String". The "/signature/f2d7c7c66450b169.html"
part of the error message is the cached page.

Here's my sweeper:
class FooSweeper < ActionController::Caching::Sweeper
  observe Foo

  def after_save(foo)

  def after_destroy(foo)

  def expire_cache(foo)
    expire_page signature_path(:opaque_id => foo.opaque_id) # line 13
    expire_page signature_image_path(:opaque_id => foo.opaque_id)


The signature_path(:opaque_id => string) generates "/signature/
string.html" route, so this part works correctly.

Here's stack trace:
#{RAILS_ROOT}/app/sweepers/foo_sweeper.rb:13:in `expire_cache'
#{RAILS_ROOT}/app/sweepers/foo_sweeper.rb:5:in `after_save'
.../app/controllers/foo_controller.rb:61:in `unsubscribe'

Any ideas what can be wrong? I'm using Rails 1.2.6.
Thanks in advance

Solved it myself - just changed expire_page to
ApplicationController.expire_page. I'm not sure why it didn't work
before - I was using Railscast 89 - "Page caching" as the reference.