When you "Add patrons" in a "Patron list", there is a "Patron search" box which does an API search. If the patron found does not have a cardnumber, a "null" value is used instead and adding the patron to the list silently fails. In 2025, many libraries don't use cardnumbers anymore. Cardnumber is also a nullable field in the database, so it does not make sense to use it in lieu of the real patron identifier (ie the borrowernumber). We should be using borrowernumber for this feature.