crash: assigning an "extra page" to a test that doesn't exist

e.g. for stuff written on non-QR'd paper, I tried to call them extra pages to say test 18 (which isn't uploaded). I got this crash:

Traceback (most recent call last):
  File "/home/cbm/src/plom/plom.git/plom/manager/manager.py", line 796, in doUActions
    self.unknownModel.item(r, 3).text(),
  File "/home/cbm/src/plom/plom.git/plom/managerMessenger.py", line 813, in unknownToExtraPage
    ) from None
plom.plom_exceptions.PlomSeriousException: Some other sort of error 500 Server Error: Internal Server Error for url: https://127.0.0.1:41984/admin/unknownToExtraPage