Master yi for no reason is immune to my mf ultimate?

Dont undestand why he is immortal and running at me. him running at me is like the last 5th of the video or so. but i hit him with 6 waves of my ultimate and his hp doesnt change at all. It cost us the game. lol. its just a game. But pisses you off when a 1v5 champion suddenly gains immortality for no reason.