Zombies not burning with Fire Aspect weapons

I was spawning zombies and fighting them with a fire aspect enchanted
stick. For some reason the zombie never caught on fire until I pushed it
outside.

What I expected to happen was...:
I expected the zombie to catch on fire.

What actually happened was...:
It didn't catch on fire, it just stood there.

Steps to Reproduce:
1. Get any item, maybe a sword or a random item like Stone
2. Give it Fire Aspect II enchantment via enchanting book.
3. Spawn in a zombie inside of a enclosed building with a roof.
4. Attack the zombie with your weapon.
5. No fire!