CLOB and ORA-06502 numeric or value error

I have a procedure in package returned a ora-06502 error message.

it goes something like this:

for i in (select * from table) loop
          msg := msg || ' Found column : :'||table.column;
end loop;

I checked and double checked msg is declared as CLOB; The procedure stopped at this statement and when the size of msg exceeded 32767.

However, I wrote a debug procedure in the same package. It worked OK; It uses the same variable in the package, i.e. msg.

procedure debug_test is
begin
LOOP
     msg := msg||lpad('x', 5000, '0');
     dbms_output.put_line(dbms_lob.getlength(msg));
     IF dbms_lob.getlength(msg) > 100000 THEN
        exit;
     END IF;
END LOOP;
end;

I know by using a  CAST(XXX as CLOB) will solve the problem but I am puzzled why one procedure works and not the other. Can anybody see the difference? What am I doing wrong?

Thanks in advance.