BUG or feature?

Hi.
Bug or feature?

CREATE FUNCTION upd (text, text) RETURNS text AS '
  BEGIN
    RETURN $1;
  END
' LANGUAGE 'plpgsql';

SELECT upd('aaa','bbb');
aaa

SELECT upd(null,'bbb');
<null>

All seems ok, but how about next ?

SELECT upd('aaa',null);
<null>


--
ó Õ×ÁÖÅÎÉÅÍ, ÷ÁÝÅÎËÏ íÁËÓÉÍ,
îÉÖÅÇÏÒÏÄÓËÉÅ ÉÎÆÏÒÍÁÃÉÏÎÎÙÅ ÓÅÔÉ
(8312) 30-19-05, 34-00-02, 30-09-73

With best regards, Max Vaschenko,
Nizhny Novgorod Information Networks.