MS SQL 6.5 Stored Proc with Text (Memo) Param

When I pass a NULL value to a text (please note 'text' as in text/image)
parameter in a stored proc I get the following error message:
General SQL error. [Microsoft][ODBC SQL Server Driver][SQL Server]
Operand type clash: image is incompatable with text.
If I pass a non-null value everything works fine.  In addition, when I
pass NULL I notice that SQL trace reports the following line getting
executed:  MyProc NULL, NULL
instead of just: MyProc NULL
I can call this proc just fine using NULL and Microsoft ISQL/w.  What is
Delphi (or ODBC) doing with this NULL parameter, and how can I
effectively pass NULL to the proc?
Any help is much appreciated!