POST-RECEIVE: Triggered hook for non-existing user " "

Hi,
I am running Gitlab version 8.6.6. I am integrating the gitlab web hooks with Jenkins gitlab plugin and from time to time I got a strange jobs created. and when I checked the log for gitlab. I found this error

ERROR -> POST-RECEIVE: Triggered hook for non-existing user " "
Do you have any idea what might cause this problem ?
Also how a push from a non existing user can be accepted and triggers the webhook ?
If you can help me trace the problem, this will be helpful as welll as the log file doesn’t give much information