Hi Lexin! There is probably an out-of-gamut value in one of the fields required to generate the to-hit roll. Perhaps a space or text value in the Spellcasting score or part of the formula that creates it. It could be a malformed magic item, for instance. After casting the spell, press the up arrow in the chat input box. It should show you the formula that was sent (depending on how it was activated), and that might give you a clue where to look.