Something went wrong during merge: Invalid merge source.

Summary

Merge request fails to merge. Displays error Something went wrong during merge: Invalid merge source. and reopens immediately

Steps to reproduce

Note: Only able to replicate on this project

1. Edit any file in the https://gitlab.com/freelygive/drupal/contacts project. (Edited contacts.info.yml for this test)
2. Submit a merge request.
3. Fails to merge with above error.

Example Project

https://gitlab.com/freelygive/drupal/contacts/

original report (internal): https://gitlab.zendesk.com/agent/tickets/118222

What is the current bug behavior?

Merge request fails to merge with Something went wrong during merge: Invalid merge source. and reopens immediately.

What is the expected correct behavior?

MR should merge

Relevant logs and/or screenshots

https://log.gitlab.net/goto/e7b1398c2ed41eb7e87bf01436afd186

Output of checks

This bug happens on GitLab.com