autoexecute and NULL values

In learning to PEAR DB, I am stumped on how to use autoExecute() for INSERT or UPDATE when some fields should be set as NULL in the database record. If I leave the field empty, this does not translate to NULL in the actual query generated yielding an error when a value is expected. I could simply check each field set in the autoExecute() fields array before including it but I'd expect that there is a more elegant solution. Any help appreciated. TIA -- Kevin McCarthy kemccarthy1@gmail.com