Card Import Update not Removing Existing Data

I'm trying to use the import assistant to update the 3 Custom Fields in existing Card records (both Customers and Suppliers in separate CSV file imports).  I'm matching cards using their Record ID (obtained from previous exports).  Some cards have existing data in the Custom Fields that need to be removed while others just need to be updated.

After the imports I find that that those fields that needed updating were updated successfully but where there is existing data that needed to be removed,  i.e. the corresponding column of the import file was left empty, the existing data is not removed.  No errors were reported.