Sion ult vs lulu polymorph

I am a sion main, and lulu is pretty strong right now, meaning I have been encountering this a lot. Sion's ult clearly states he is immune to all forms of CC, and lulu's polymorph is classified as a form of CC. It would be great if either a clarification can happen or this be fixed if it is a bug. If it is supposed to happen, I can play with it in anticipation of it, but I can't if it is just a bug.