diff --git a/actionpack/lib/action_controller/caching/sweeping.rb b/actionpack/lib/action_controller/caching/sweeping.rb
index 808a6fe5f3..bb176ca3f9 100644
--- a/actionpack/lib/action_controller/caching/sweeping.rb
+++ b/actionpack/lib/action_controller/caching/sweeping.rb
@@ -54,6 +54,11 @@ module ActionController #:nodoc:
       class Sweeper < ActiveRecord::Observer #:nodoc:
         attr_accessor :controller
 
+        def initialize(*args)
+          super
+          @controller = nil
+        end
+
         def before(controller)
           self.controller = controller
           callback(:before) if controller.perform_caching