Not able to Devour an ally as Tahm Kench

I don't have a proof of this. and I tried replicating it in a custom game but it didn't work. While playing a normal game with friends, I was not able to eat the Malzahar on my team, my W worked fine on other allies. on Malzahar the cursor turn green but nothing happen and the ability does not go on CD. Anyways am just posting this to see if someone had similar issues.