Cannot close merge requests.

Summary

I cannot transition Merge Requests into the Close state.

Steps to reproduce

0. Open a Merge Request.
1. Press the Merge button to approve the merge request.
2. Then at the bottom of the Merge Request page, enter some comments, and press the Comment and Close Merge Request button.
3. Notice that an error surfaces at the top of the page, and that the Merge Request in fact does not close.

See attachment with screenshot for the error.

Example Project

https://gitlab.com/cread20200228/give-it-a-spin

What is the current bug behavior?

Results in error.

What is the expected correct behavior?

Closes Merge Request with no error.