[Bug] Karthus Ult Didn't Give Assist On Tahm Kench kill

Played Karthus against Tahm. Tahm's teammate is low so I activated my ult. As soon as I activated my ult Tahm swallowed his teammate, his teammate was saved by Tahm but Tahm died soon after to my team. I didn't get any assist for the Tahm kill. WTF?