is nunu suppose to be able to q xerath?

Been playing nunu today for the first time in ascension and after i q'd xerath to secure it, someone said that's cheating and reported me after game. Saying something along the lines of one of xerath's passives is suppose to block nunu's q and cho'gath's ult. Just wondering, is this a glitch? Or is it allowed? And am i going to get banned for this?

Any help would be greatly appreciated, as i dont want to be doing something that'll get me punished