Mordekaiser ultimate prevents undo

Alright, i played Mordekaiser, i ulted someone and then died a few seconds later. While dead i used shop to buy an item, which happened to be a missclick and i tried to undo the purchase. Unfortunately the ultimate was still active and i guess that registered as doing damage with the new item, so i was unable to use the undo button. This may or may not be a bug, but i still feel it should be removed from the game.