I was writing some program that uses the WM_CONCAT function. When I run this query:

SELECT WM_CONCAT(DISTINCT employee_id)
FROM   employee
WHERE  ROWNUM < 20;

It works fine. When I try to compile the relatively same query in a package function or procedure, it produces this error: PL/SQL: ORA-30482: DISTINCT option not allowed for this function

FUNCTION fetch_raw_data_by_range
RETURN VARCHAR2 IS

    v_some_string VARCHAR2(32000);

BEGIN

    SELECT WM_CONCAT(DISTINCT employee_id)
    INTO   v_some_string
    FROM   employee
    WHERE  ROWNUM < 20;

    RETURN v_some_string;

END;

I realize WM_CONCAT is not officially supported, but can someone explain why it would work as a stand alone query with DISTINCT, but not compile in a package?