order by in cursor declaration does not allow update

If a cursor is declared using "Order by" then it gives following error
during updation of the cursor:
ERROR: cursor "c" is not a simply updatable scan of table "test"
Ex:
DROP TABLE IF EXISTS test;
create table test (num int,num2 int );
insert into test values(1,100);
insert into test values(2,200);
insert into test values(3,300);
insert into test values(4,400);
insert into test values(5,500);
BEGIN;
DECLARE c CURSOR FOR SELECT * FROM test ORDER BY num;
FETCH 2 FROM c;
UPDATE test SET num = 500 WHERE CURRENT OF c;
ERROR: cursor "c" is not a simply updatable scan of table "test"
SELECT * FROM test;
FETCH 2 FROM c;
COMMIT;
SELECT * FROM test;
FETCH 2 FROM c;
COMMIT;

Comments for this...??

Regards,
Dharmendra
www.enterprisedb.com