Apphosting, trying to publish branch but I can't enter it

I am trying to publish a new revision in app hosting, but when I click create rollout and try to enter a branch name (which I deployed before) the box turns red and the create button greys out. Other than the default I can’t type anything into that box that works - tried clicking set as default etc, but nothing un-greys the create button.

Anyone else having this issue?