Wither skulls CANNOT be summoned with /summon

When I was on my creative world, I was doing some stuff with Wither Skulls and it wasn't working. When I looked at the commands, I realized the skulls themselves weren't being summoned. I tested how to summon them, and couldn't find a single way!

What I expected to happen was:
For a wither skull to be summoned.

What actually happened was:
Wither skulls weren't being summoned.

How to reproduce:
1.Summon a wither skull in every way you know using /summon.
2.See that it is impossible to summon wither skulls!