backup_import_questionbank prevents quiz duplication when turned off

1. Browse to "Site administration" > "Courses" > "Backups" > "General import defaults"
2. Disable backup_import_questionbank
3. Create a course and add a quiz to it
4. Navigate to the course you containing the quiz you just made
5. Attempt to duplicate the quiz (edit menu > Duplicate)

The duplication fails, with no indication why. The user should at least be notified somehow of what they need to do in order to get the quiz to duplicate.