Pushed WitherSkulls become invisible

Now when you summon a witherskull with this command:

/summon WitherSkull  ~ ~ ~ {direction:[]}

You can actually see it, but when you push it with a piston, it becomes invisible, but is still there.
How to test:

/testfor @e[type=WitherSkull,r=5]

When you reload the world you can see them again.