Problem with FOR UPDATE

Hi All,
I have a query in which I want to SELECT FOR UPDATE same rows but only 
from one table.
Firs I try just with SELECT FOR UPDATE but I receive an error
because of the LEFT JOIN - "ERROR:  SELECT FOR UPDATE/SHARE cannot be 
applied to the nullable side of an outer join".
So I decide to use SELECT FOR UPDATE OF <table name> but I then receive 
the error you can see.
Can anyone help me with this query?

Thanks in advance.
 Regards,   Kaloyan Iliev

rsr=# SELECT
rsr-#        DD.*
rsr-#                                         ( SELECT sum(-amount * 
saldo_sign(credit))
rsr(#                                                   FROM acc_debts ACD1
rsr(#                                                   WHERE 
ACD1.debtid = DD.debtid ) AS saldo,
rsr-#                                                 C.custid,
rsr-#                                                 S.descr_bg
rsr-#                                FROM debts_desc DD LEFT JOIN config 
C ON (DD.conf_id = C.id),
rsr-#                                         acc_debts AD,
rsr-#                                         acc_clients AC,
rsr-#                                         services S
rsr-#                                WHERE DD.debtid = AD.debtid
rsr-#                                       AND DD.closed AND NOT 
DD.paid AND DD.has_invoice AND DD.active AND DD.has_proform AND NOT 
DD.storned
rsr-#                                      AND AD.transact_no = 
AC.transact_no
rsr-#                                                                  
AND AC.ino = 45
rsr-#                                                    FOR UPDATE OF 
debts_desc;
ERROR:  relation "debts_desc" in FOR UPDATE/SHARE clause not found in 
FROM clause
rsr=# select version();                                           version
------------------------------------------------------------------------------------------------PostgreSQL 8.1.4 on
i386-portbld-freebsd6.1,compiled by GCC cc (GCC) 
 
3.4.4 [FreeBSD] 20050518
(1 row)