SQL & PL/SQL

Dear I am trying to impliment a VPD based security with Apex

The function I have used is

create or replace
FUNCTION CC_SEC (
p_schema IN VARCHAR2 DEFAULT NULL,
p_object IN VARCHAR2 DEFAULT NULL)
RETURN VARCHAR2
AS
begin
--RETURN 'upper(userid) = nvl(v(''APP_USER''),USER)';
RETURN '(upper(userid) = decode(nvl(v(''APP_USER''),USER),'||'''ADMIN'''||',upper(userid),nvl(v(''APP_USER''),USER)';
END;

and the add policy is as below

BEGIN
DBMS_RLS.add_policy
(object_schema => 'TMAPP',
object_name => 'USER_DST',
policy_name => 'DATA_SEL_POL',
function_schema => 'TMAPP',
policy_function => 'CC_SEC');
END;


here in the function cc_sec, I am tryin to use the decode option. If am using with out decode (the line is commented above), it will work without any issues. But after putting the decode I am getting the error ORA-28113: policy predicate has error from the apex login.

Please help

Dennis