I need to catch the following error ORA-01442

In my script below, does it need the leading zero? -01442 or -1442 which of the two values and why?

----------------------------------
    -- Alter column null --
    ----------------------------------
    DECLARE
       allready_null EXCEPTION;
       PRAGMA EXCEPTION_INIT(allready_null, -01442);
    BEGIN
       execute immediate 'ALTER TABLE NMSACTIVECONTACT MODIFY ("ISOURCEID" NULL)';
    EXCEPTION
       WHEN allready_null THEN
          dbms_output.put_line('ISOURCEID is already null, skipping...');
    END;
    /

It came to my attention when it fail to catch the exeption because the error code was different.