Karthus's dead body is terrain?

I recently played vs a Karthus and after his passive faded (so when he was actually dead) the big black circle on the ground where his dead body should be is impossible to move through, it's like a wall or minion block or something, I recreated this in a custom game where I killed Karthus and other champs over and over and from what I tested it only seems to happen with Karthus. Not sure if this is intended but If so I haven't noticed it before