Fire Aspect Sword won't light mobs on fire.

I was under attack by a horde of zombies. My sword had Fire Aspect II on it, so I struck them and...
They weren't lighting on fire. At all. Once I discovered the glitch, I tested it in both Survival and Creative, and it didn't work in either. I did notice, though, that when I kill animals with the Fire Aspect Sword, they still dropped cooked meat like normal.