Thresh Relic Shield

Tried to play thresh since he if ftp this week. Bought relic shield, but the passive to kill minions below 200 + ad health would not work. It would still give gold to the adc, but only if i was able to last hit the minion with my regular attack damage. Problem still continued when i upgraded to targon and face of the mountain. Played a game right after with a mundo support and the item worked normally.