SQL & PL/SQL

We have a situation where we need to allow particular users which are stored in a table.

create table ALLOW_USERS (username varchar2(30) );

CREATE OR REPLACE TRIGGER USERS_TRIGGER
   AFTER LOGON
   ON DATABASE
DECLARE
BEGIN
  IF DBMS_STANDARD.LOGIN_USER NOT IN (SELECT USERNAME FROM ALLOW_USERS)

	    THEN
		     RAISE_APPLICATION_ERROR (-20001, 'Unauthorized login');
	  END IF;
END;
/

Warning: Trigger created with compilation errors.

SQL> show error
Errors for TRIGGER USERS_TRIGGER:

LINE/COL ERROR
-------- -----------------------------------------------------------------
3/3      PL/SQL: Statement ignored
3/38     PLS-00405: subquery not allowed in this context

How to use subquery on the above trigger ? or is there any better way to achive required results .

-Thanks