Cannot create MR's from forked branch

When I go to my forked working branch to create a MR upstream, I get a weird form error I've never seen before, disabling me from continuing.

I.e.

1. Click Create MR from my own fork branch.
2. Edit the title, write a desc, Assign to Me, and then click Submit MR [there is automatically one approver setup (required shows 1, then there is one listed)]
3. Taken to a page (never seen this before) saying New Merge Request showing a source/target branch box with dropdowns. Under those is the following:

Clicking the button takes me back to the page prior. Trying again still fails. This is happening on Gitlab.com specifically (8.9.0-ee eb7f876).