Can not activate merge request approvals for the project

Summary

When I try to activate merge request approvals for the project from 'Project settings' page and save, got 'Approver groups is invalid'.

Steps to reproduce

  1. Open 'Project settings' page - https://gitlab.com/<group_name>/<project_name>/edit
  2. Check 'Activate merge request approvals'
  3. Add a few persons from the team, who works with this project.
  4. Click 'Save Channges'

What is the current bug behavior?

Shows msg: "The form contains the following error: Approver groups is invalid"

What is the expected correct behavior?

Update settings

Relevant logs and/or screenshots

Screen_Shot_2017-04-26_at_2.33.42_PM

Output of checks

This bug happens on GitLab.com

What's interesting

If I go, from settings page to project page - https://gitlab.com/<group_name>/<project_name> and then come back to settings, and repeat all steps, it works fine.