Stuns not affecting Garen when he ults [GAMEPLAY]

Playing as Xerath my E ability stuns opponents. In this case my stun is not working. This garen on the other team is coming in for a kill and I use my stun ability on him, he gets hit but then he uses his ultimate and the stun is gone he should be stunned. this is a problem because the stun is xeraths best escape and support move and when it doesn't work its annoying. please fix this bug. I'm not sure if it is with any stun or only xerath's but its not good.