how come there was "caii nu au apă" as a wrong answer for the multiple choice (like when you type it like that in the regular question it still accepts it because it can't penalize you for not having an accent)

Is there a rule for when you can shorten the word "nu"? Like in "Caii n-au apă." If I had to take a wild guess, I'd say only when the following verb starts with a vowel?