[GAMEPLAY] Interaction between Zac and Caitlyn

I was playing Caitlyn, and a Zac used Elastic Slingshot on me, but he didn't actually hit it. Meanwhile, i was ulting, and it cancelled me ult. I clipped this, and went through it frame-by-frame. I start ulting, Zac lands, my ult finishes the cast time, my ult doesn't go off, goes on the 5 second cooldown, I take the damage from e, but instead of getting knocked back, I just simply walk away. I tested using Zac's e, and Cait's ult separately in practice tool, and couldn't come up with the same result with either of them. http://plays.tv/video/59dc4b2db1be30a16a/weird-interaction