Lee Sin ult and Vladimir pool bug [GAMEPLAY]

I'm pretty sure that this is not supposed to happen, but I was playing a game as Vlad, and when the opposing team's Lee ganked from behind, as soon as I got in ranged of him I decided to pool so that he couldn't ult me. Well, right as I was pooling, Lee ulted and I stood in place not being able to do anything for the duration of how long the kick from Lee would normally be. Then I when into pool form and was able to move again. This has only happened once which was today.