Upstream not listed as target for merge request

This is my first attempt at creating a merge request on Gitlab so perhaps I’m doing something wrong here since this is very basic functionality. I forked a repository (DavidGriffith/xv) and pushed a change to a new branch. Then, when creating a merge request, I can see my own master and sibling forks but upstream is not listed there. How do I create a merge request for upstream master?