[Game] Teemo cannot undo an item

Putting it very simply, i was playing a ranked game when i got 2v1'ed and died. I accidentally bought a BoRK instead of a runaan's. I went to click the undo button and it was greyed out. The only other time it is grey is if it was a potion last bought. I assume the poison is the reason, because I'm "in combat" I have tested it, and the same thing will happen. Auto someone as teemo, die and immediately buy an item.