Oracle Database Discussions

I have a problem, trigger has been created but

SELECT sys_context('USERENV', 'MODULE') FROM dual;
this above query is showing blank.

I dont have much knowledge about dbms_application_info please help.

Trigger:

CREATE OR REPLACE TRIGGER afterlogon
AFTER LOGON on DATABASE
BEGIN
IF upper(substr(sys_context(''userenv'',''module''),1,3)) ='SQL' THEN
RAISE_APPLICATION_ERROR (-20001, 'You are not allowed using this application');
END IF;
END;