dbms_server_alert.set_threshold - set instance_name to null

I have been trying without success to set the instance_name to null or to a shorter string in dba_thresholds.  All of the following statements succeeded (PL/SQL procedure successfully completed.), however the instance_name was not changed. What am I doing wrong?  Please advise.  Thanks.

EXEC dbms_server_alert.set_threshold (1000, dbms_server_alert.operator_GT, '10', NULL, NULL,1, 1, NULL, dbms_server_alert.object_type_EVENT_CLASS, 'Application');

EXEC dbms_server_alert.set_threshold (1000, NULL, NULL, NULL, NULL,1, 1, NULL, dbms_server_alert.object_type_EVENT_CLASS, 'Application'); EXEC dbms_server_alert.set_threshold (1000, NULL, NULL, NULL, NULL,NULL, NULL, NULL, dbms_server_alert.object_type_EVENT_CLASS, 'Application');