SQL & PL/SQL

Hi Gurus,

      I am trying to understand if an oracle function return NULL as a return?

      I have a function which returns null and I can invoke it in a select without error.

      Somewhere I read function shouldn't return a null or would get an error if used in a 'select statement' if it returns null.

      I just want to confirm if I am missing anything.

CREATE OR REPLACE FUNCTION MY_FNC(in_param IN varchar2) RETURN VARCHAR2 IS
         A VARCHAR2(30);
BEGIN
       SELECT O.OWNER INTO A  FROM ALL_OBJECTS O WHERE O.OBJECT_NAME = 'a';
       return a;
      DBMS_OUTPUT.PUT_LINE('a: '||a);
END;

select my_fnc('a') from dual;    

Thanks