how to use a cursor for update?

Hi all,

I'm trying to create a stored procedure using a cursor to udate a
table this is my code:


CREATE OR REPLACE FUNCTION test_select() RETURNS void AS
$BODY$

DECLARE
    cur CURSOR FOR SELECT *
               FROM zone
           WHERE patient = '4595'
           FOR UPDATE;
    var1 record;
    var2 record;
BEGIN
    OPEN cur;

    FETCH  cur INTO var1;

    FETCH cur INTO var2;

    UPDATE zone SET end_period = var1.end_period WHERE CURRENT OF cur;

END;
$BODY$
LANGUAGE 'plpgsql';


When i try to execute this code i get a :

ERROR:  cursor "cur" is not a simply updatable scan of table "zone"
CONTEXT:  SQL statement "UPDATE zone SET end_period =  $1  WHERE
CURRENT OF  $2 "
PL/pgSQL function "test_select" line 16 at SQL statement

How can i use that cursor for update?

Tnx in advance!