Damaging Tahm Kench's shield does not grant assist

Title is pretty self explanatory; I've observed this in a recent game. If Tahm Kench uses Thick Skin and you only manage to deal damage to his shield (and not his health) you will not be granted an assist. This sucks because it's easily the biggest shield in the game and can soak up ultimates-worth of damage so I can't believe this is intended. A response from Riot would be great :)