Summon a wither skull to an arrow doesn't work.

When summoning a wither skull onto an arrow it is unable to do so which has worked in previous versions,

What I expected to happen was...:
A wither skull should be summoned to the moving arrow

What actually happened was...:
no wither skull appeared an I got an error message which read the following: [17:13:08] Failed to execute 'summon WitherSkull ~ ~ ~' as Arrow

Steps to Reproduce:
1. Create a simple but fast redstone clock hooked up to the command: /execute @e[type=Arrow] ~ ~ ~ summon WitherSkull ~ ~ ~
2. then fire an arrow, what should happen is a wither skull should 'ride' the arrow but it can't summon
in previous versions this would fire a wither skull on an arrow but is unable to summon now