Tahm Kench can't Devour Sion

Description: As Tahm Kench, you can Devour any champion if they have three stacks from his attacks correct? Well there is a bit of a problem I ran into. When Sion dies and reanimates due to his passive, Tahm cannot Devour him at all. I was playing with my team and we won a team fight, but needed to deal with an undead Sion. So I hit him with a basic attack, a Q, and another basic attack having 3 stacks. But what happens is when I press W to devour, Tahm still does his basic attacks to Sion. Now im pretty sure Sion isn't invulnerable to status effects (Bard ult and Ahri's Charm for example.) And im sure he is still a champion and nothing else (Considering he can still get kills in his undead state and added to his score.). Tahm Kench's W should be able to Devour him with full stacks. Even if Sion is undead. If this is not a bug, I think Tahm's W description should be remade to include exceptions.

Steps: Step 1: Have a Sion (With High amount of health so he wont decay quickly) die and reanimate Step 2: Have Tahm Kench quickly add three stacks to Sion Step 3: Try to Devour

Now I am not sure if this is a one time thing or not, but I am too worried for the community if Tahm isn't able to devour Sion because of this bug. (If it is one.) This may cause people to lose fights they could've won. I hope this bug will be able to be addressed soon.