Malphite Ult cannot be cast following hard CC

I've noticed this (and died because of it) a number of times. As close as I can tell, when playing Malphite, after you've been hit with hard CC, you can't cast your ult for a period of time after.

Steps to reproduce: Be Malphite at level 6, have someone on the opposing team have hard CC (stun, knockup, etc). Get hit with the CC, and attempt to ult.

I don't know the exact period of time, but it's more than a second after the CC wears off. I can perform any other activities during that time (like Flash), but I can't use his ult.