You’re dead on, @owen.campbell – fix now deployed!
Unfortunately, we’re currently tied to using master as our primary branch, so the workaround is to create a synthetic master branch on clone (it won’t be pushed back if you don’t touch it!). But at least you can edit the repos now 
(Yes, we do intend to remove the exact-name dependency on "master" branch, but there are many direct references to that name in infrastructure supporting the Classic Editor – which means that this would requires non-trivial changes to legacy code that’s due for retirement in the forseeable future. That makes it a rather unappealing use of resources!)