Can't summon certain things.

The summon command can't summon WitherSkulls/Fireballs. Even when i give them a directional tag.

I typed /summon WitherSkull ~ ~ ~

{direction:[]}

and expected to see a witherskull flash and become invisible but still be there.

It said "Unable to summon object"

Steps to Reproduce:
1. Simple enough: /summon WitherSkull/Fireball ~ ~ ~