I tried "Your math is wrong." as a rejection reason. For reference, I was rejecting an edit that said "because n+n/2+n/4+... = 2n-1", which is wrong. So I rejected the edit because the math was wrong.
This was rejected so I just went with "invalid edit."
Note I don't know the logic behind this. The only related questions I could find made it coming up at all sound like a bug. If my edit reason was invalid according to the system (it's not), I'm going to need some clarification so I can
get around this requirement submit lovely rejection reasons in less time than it takes for someone else to reject for an uninformative standard reason.