Can't filter assignees of issue

Summary

Got 500 server response when filtering assignees of issues

Steps to reproduce

  1. create new issue
  2. click assignees
  3. type keyword of user, the loading indicator shows forever

found below error if opened dev tool of chrome,

What is the current bug behavior?

Can't filter the user list

What is the expected correct behavior?

found the users by keywords

Relevant logs and/or screenshots

image

Results of GitLab environment info

official gitlab ee docker image

GitLab 10.6.2-ee GitLab Shell 6.0.4 GitLab Workhorse v4.0.0 GitLab API v4 Ruby 2.3.6p384 Rails 4.2.10 postgresql 9.6.8 Gitaly Servers