[GAMEPLAY] Cho'Gath not silencing Twitch

I was playing Dynamic Queue with a friend of mine (he was Cho'Gath, I was Thresh), and I noticed that any time he went to use his Feral Scream on twitch, it wouldn't work. I don't know if there's a specific reproduction method, but it happened virtually every time.

Link to Match History: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2121659153/213511126?tab=overvieww