Odd Error: Month must be between 1 and 12

[Andrew Rivers <andrew.rivers@CWCOM.NET>]

Hi
I get the error:
Error on line 1377:
   (SQR 1925) Month must be between 1 and 12.

Line 1377 is as follows (all on one line):
    show 'invoicenum: ' &invoicenum ' - Invoicedate: '  &invoiceDate

The problem is a NULL value for &invoiceDate in the database.
     select invoicenum, invoicedate from invoice
            where invoicenum = '1052' and invoicedate is null;
This returns:
     INVOICENUM     INVOICEDATE
     ========== ====================
     1052

     1 ROW SELECTED
showing that invoicenum = '1052' is NULL.

However, if the following code is run:
      if  isnull( &invoiceDate )
         show 'invoicenum: ' &invoicenum ' is NULL'
      else
         show 'invoicenum: ' &invoicenum ' - Invoicedate: '  &invoiceDate
      end-if

the else statement is executed for invoicenum = '1052' and the program
terminates at:
      invoicenum: 1052 - Invoicedate:

Other fields including date fields do not give problems.

If anyone has any suggestion as to the problem, I would be most grateful.

Andrew