Lulu W vs Garen Q

How come Lulu's W does not stop the damage (and effects) from Garen's Q? If Garen presses Q and then Lulu W's him (animation goes off), Garen will still be able to do damage while polymorphed. At first I thought it was a fluke but it has happened to me several times in two games. I thought polymorph didn't have a cast time, but it sure feels like it even through the animation goes off right away.