I wrote 'Dacă ai alergat, nu bea apă rece.' and it was corrected to 'Dacă ai alergat, nu bei apă rece.' Doesn't the second person singular negative imperative always use the infinitive?
Yes, it does. Your version is correct.
I had the same response. Still being marked wrong. I just reported it.
The sentence was removed and corrected in the new version of the course, thank you @gplano, @Lurch_M and @cheerfulcharlie for all your great support !!!