Removing the WIP prefix raises a 500 error

Summary

I clicked on removing the WIP prefix on MR !3188 (merged) and this raised a 500 error. Reloading the page then showed the removed WIP prefix.

Steps to reproduce

1. Edit the title of !3188 (merged) to contain the WIP: prefix
2. Save the merge request
3. Try to remove the WIP via https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3188/remove_wip