Lich King weapon does not work

I just played a game in wild. The Lich King gave me Frostmourne. I played against a warrior and killed three minions with the weapon. The last minion i killed with the weapon was Restless Mummy (still had reborn). After I killed the minion, nothing happened, although the weapon should have given me 3 minions I killed with Frostmourne previously.
This game resulted in a very frustrating loss.