[GAMEPLAY] Cassiopeia poison going through Poppy ulti

When you put your ultimate on someone as Poppy you become invulnerable to ANY damage coming from another champion. However it seems like it doesn't work for at least Cassiopeias poison.

1.Ult any ennemy champion but Cassiopeia
2.Get poisened by Cassiopeia
3.Die miserably

It happened to me during my last game with but I hadn't time to see if it's only when you get poisened by a Cassiopeia or by any other poison/DoT