Create a new coures. Go to Qusetion bank -> Categories Create a new category, with idnumber '0' (that is, a single digit zero). Verify (e.g. by looking in the database, or by having the fix for MDL-66553 , or by re-editing) that the idnumber was saved. Delete that new category. Now edit the category "Default for [_test course shortname_] " and try to set the idnumber to 0. Verify that the idnumber was saved. Edit the same category and without making any changes, save. Verify the id number is still 0 Now, edit the same category and change the 0 to 123 and save. Verify the id number was saved and is now 123 Edit the same category and without making any changes, save. Verify the id number is still 123 Now, edit the category again, but this time clear the id number field and save. Confirm the id number has now been unset and is empty.