Wither Skeletons not spawning on Wither Roses

What I expected to happen was...:
Wither Skeletons being able to spawn on Wither Roses

What actually happened was...:
No mobs spawning on wither roses

Steps to Reproduce:
1. Find a fortress with favourable spawning for Wither Skeletons
2. Prepare an enclosed area (around 34blocks square and 3 high) with Wither Roses and light up surrounding areas
3. Stand at spawnable distance.