Got "Failure to squash" error when trying to merge an MR that had 1 commit

This merge request (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25266) had the "squash commits" checkbox checked by the author, but the MR ended up only having 1 commit. When I went to merge it, I got this error:

After unchecking the box myself, I was able to merge the MR successfully 🤷