API - Sort merge requests issue

When trying to sort merge requests according to MERGED_AT_ASC or MERGED_AT_DESC, I receive an “Internal Server Error”.

Note that if I sort the merge requests according to UPDATED_DESC for example, the response is good.

Any idea how to sort the merge requests according to their merge date?
I guess it’s a bug but I can’t open a ticket to the GitLab support as a free trial :frowning: