Veigar Ult (double-cast) doesn't seem to work on Berserkers

I had Level 3 veigar and I turned my volibear into a spell critting mage for 6 mage buff (that execute giving him full mana along with double cast is pretty legit)

But whenever my tier 3 veigar ulted with double cast against a target berserker, regardless of their level, the doubled ability would not damage or insta-pop its target.

I got 7th place in a match where this Veigar should have delated pretty much entire teams single-handedly, but didnt damage Berserker units (I saw it on Renekton, Jax, and Olaf.)