Zombie spawn eggs do not spawn zombie villagers

To reproduce this bug, use zombie spawn eggs to create dozens of zombies. I expected a few of these zombies to be zombie villagers, but this was not the case.

Monster spawners appear to work fine; standing next to a zombie spawner (or cluster of zombie spawners) will eventually spawn a zombie villager.

 

Edit: It appears that zombies and zombie villagers are internally seperate mobs, so this my be intentional.