Unfortunately, you can't rely on people always doing "the sensible thing", for any number of good reasons. If a particular set of parameter values didn't cause failure, it is probably used somewhere.

Anyway, your problem goes away if interval == 1, right? If we were to tighten things up (e.g. disallowing interval > 1 with "midnight"), then it would have to be done on a deprecation cycle at the very least, ISTM.