I actually tried to use that test, just modified slightly for this new type, but was never able to get the test to fail.  I tried tinkering with it a bit, but either I was incredibly unlucky or I wasn't making the right adjustments.  I'll try some more and see if I can get it to fail.  If I can't get the test to fail, is the preference to omit the unit test entirely since if the test can't verify, what good is it?