auto #valid? good or bad idea for tests?

I've recently added this to the bottom of my test_helper.rb (it should
really be in its own file, but first I want to get feedback):

module ActiveRecord
  class Errors
      alias_method :old_on, :on
      def on(attribute)
        self.instance_variable_get("@base").send :valid?

This alows me to avoid calling #valid? manually in every test where I
am checking for a validation error:

topic =
assert_equal topic.errors.on(:title), "must be set"

Instead of:

topic =
assert_equal topic.errors.on(:title), "must be set"

I know that doesnt look like a huge deal, but when you have many tests
which need to do something like this all the #valid? calls get ugly
and repetetive. So, I'm looking for a reason anyone may have as to
why this would be a bad idea. Im hoping no one can come up with
reasons why it would be a bad idea, but if there are some I'd rather
find out now.