Error creating patch queue for private repo (BB-9128)

On a private repo (which does not allow public forks), click "fork" then "create a patch queue". The checkboxes for "this is a private repo" and "disallow public forks" are disabled (and appear to be checked), but when you submit the form it fails with the message "public repos must allow public forks".

Using chrome's dev tools to remove the disabled attribute from the two check boxes allows the patch queue to be created, so the problem is in the web UI rather than the backend.