Dashes in tag-names fails in non-obvious ways

I tagged my last topic with "sign-up" and "sign-in" and was presented with a relatively obscure error message: "tagging is invalid. tagging is invalid."

Being a developer and all that, it wasn't hard to guess that the double error was triggered by two dashes. But I'm sure the message could be nicer and more obvious (even just some in-place help text like "tags can only use letters and spaces")