I don't know what do you mean with "proper", but in my opinion this is not wrong way to say it in this case, of course there are other ways to say it, but this is shorter and commonly used in other translations. Maybe would be better with  "Полето :attribute е в невалиден формат." or "Полето :attribute е с невалиден формат." instead of  "Стойността на :attribute е невалидна."?