Tahm Kench and Riven Bug

So i just played a game as Tahm Kench to have some fun playing some out of meta picks. As the game progressed, i tried to devour the Riven on my team but i was unable to. At first i thought i was misclicking and i was just bad. After i realized what was happening, in the same game i tested to see if i could devour the Riven, but i was unable to. I am not sure what caused the bug if anything but i was not able to devour her for the enitre game.