[GAMEPLAY] On Tahm Kench, can't devour Kog'Maw?

So, I was playing as Tahm Kench support with a buddy who was playing as Kog'Maw. At some point I noticed I could not use my W - Devour on him. It worked on any other player, be it friend or foe, but him specifically - it was a no-go. This lasted the entire game.

As to recreating the situation - there's nothing to it, really.

1. Try to cast W - Devour on Kog'Maw
2. Nothing happens
3. Feel like a terrible support for not doing that one thing you're supposed to do (I had one job...)

The only difference (that I can think of) between Kog'Maw and the rest of the players was that I was premade with him.

Thanks! SurfingBird