Tear goes on CD when initially purchased after one spell

This happens to me fairly often. If I buy a tear (only tested on Heimer, presumably the same with all champs) and then cast a spell, tear will go on CD as if I had cast two spells. It's not a big deal, but I like to cast EW in the fountain so that my mana recharges as I'm leaving. If I wait a couple of seconds then it acts as normal, so I'm thinking that maybe purchasing it counts as a spell, but doesn't increase the mana on it.