Killing a Warden doesn't trigger the Monster Hunter advancement

The Bug:

Killing a Warden doesn't trigger the Monster Hunter advancement.

Steps to Reproduce:

Spawn a Warden, and kill it using any Weapon. 

Observed Behavior:

The Warden dies, but it doesn't grant you the Monster Hunter advancement.

Expected Behavior:

The Warden dies and you should get the Monster Hunter advancement.