How do I unit test this?

I've got a cache sweeper running observing a model
"after_validation_on_update". Inside that filter method, a call to
"controller.current_user" is made to access that
application_controller method (to get the current user in session).

I get the error "NoMethodError: undefined method `current_user' for
nil:NilClass" when running my unit test because, by definition, there
is no controller instantiated in a unit test. How do I get around
this to test? Thanks for any help.