How can I use a 'expire_fragment' in ActiveRecord

I must use a 'expire_fragment' in ActiveRecord, I just new an
ActionController, and use the expire_fragment, but it doesn't work if
I use a style like
expire_fragment(:controller=>'product',:action=>"xxx",......)

this is the error :

/home/zhixin/lib/ruby-1.8.6/lib/ruby/gems/1.8/gems/actionpack-1.13.3/
lib/action_controller/base.rb:522:in `url_for'
/home/zhixin/lib/ruby-1.8.6/lib/ruby/gems/1.8/gems/actionpack-1.13.3/
lib/action_controller/caching.rb:326:in `fragment_cache_key'
/home/zhixin/lib/ruby-1.8.6/lib/ruby/gems/1.8/gems/actionpack-1.13.3/
lib/action_controller/caching.rb:375:in `expire_fragment'
lib/cache.rb:19:in `expire_product'
app/models/agency.rb:43:in `after_save'
app/models/agency.rb:41:in `each'
app/models/agency.rb:41:in `after_save'
app/controllers/user_controller.rb:182:in `update_profile'

this is the code:

      a = ActionController::Base.new

a.expire_fragment(:controller=>'product',:action=>'full_product_detail',:id=>id,:to=>'false')