Using the COUNTRIES table as a parameter

One of my clients likes to use the COUNTRIES table as a parameter to reports. Unfortunately, the COUNTRY field in the CUSTOMERS table is not mandatory, so there can be customers without a defined country.

Running a procedure with this parameter set to a specific country works correctly - customers with that country are displayed. Running the procedure with country set to * returns all the customers that have a defined country. What about those customers without a defined country? Leaving the parameter empty apparently causes the error "String or binary data would be truncated (Error 8152)". I don't understand why this error message should appear.

In the past, I might have suggested 'solutions' such as setting the parameter to be 'not equal to Guadeloupe', in the hope that no existing customer is situated in Guadeloupe (where is Guadeloupe? It's "an overseas department and region of France in the Caribbean").

Yesterday my brain must have been working correctly as I saw a much better solution: define the parameter as type NFILE instead of FILE. Add to the code  this section

LINK COUNTRIES TO :$.COU;
ERRMSG 1 WHERE :RETVAL <= 0;
GOTO 1 FROM COUNTRIES WHERE COUNTRY > 0;
UNLINK COUNTRIES;
LABEL 1;

This means that if the parameter is left empty, the COUNTRIES table will be unlinked and the following query will use the original COUNTRIES table, in which the empty value exists. Indeed, after changing the parameter's type, running the procedure with the COUNTRIES parameter left empty worked perfectly.