How to pass NULL in where array for $wpdb->update

My simple and quick solution is the use of a normal $wpdb->query() function:

$wpdb->query( $wpdb->prepare("UPDATE $schedule_table SET rpt = %s WHERE post_id = %d AND start IS NULL AND end IS NULL", $event_tag, $post_ID ) );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)