Error: The FilterField has an empty Filter argument...

Sometimes you want to filter an NL() function based on the contents of another cell (or the output of another NL() function) which could be a blank value.  When you run the report, #VALUE is displayed in the cell and one of two error messages (depending on your version of the Jet Excel Add-In) is displayed:

Complete text of error message is:

The FilterField {name of field} has an empty Filter argument, which is not allowed.  Use '' to filter for blank, or prefix the Filter argument with @@ if it is a cell reference.

or

Empty filter not allowed.

Cause:

The underlying database does not support blank filters.  Thus, the NL function cannot allow filters to be blank.