wither skulls disepears when it has been summoned

Put the summary of the bug you're having here

*What I expected to happen was that the wither skull should appear

*What actually happened was the wither skull just diseppear

Steps to Reproduce:
1. type this in a command block:

/summon WitherSkull ~ ~1 ~ {direction:[0.001,0.001,0.001]}
2. click done

3. push a button