Lookups fail when the string used for lookup is not correctly prefixed

[djm]

>>> from x.x import Contract
>>> Contract.objects.filter(public_id="")
...snip...
django.core.exceptions.ValidationError: ['“” requires the prefix “ct_”.']

This query should be allowed fine.

>>> from django.shortcuts import get_object_or_404
>>> get_object_or_404(Contract, public_id="some-random-value")
...snip...
django.core.exceptions.ValidationError: ['“” requires the prefix “ct_”.']

This query should be possible, else we can't correctly raise 404s.