Using index for IS NULL query

Index is not used for

 is null

condition:

create index  makse_dokumnr_idx on makse(dokumnr);
explain select
     sum( summa)
   from MAKSE
   where  dokumnr is null

"Aggregate  (cost=131927.95..131927.96 rows=1 width=10)"
"  ->  Seq Scan on makse  (cost=0.00..131927.94 rows=1 width=10)"
"        Filter: (dokumnr IS NULL)"



Table makse contains 1200000 rows and about 800 rows with dokumnr is null so
using index is much faster that seq scan.
How to fix ?

Andrus.

"PostgreSQL 8.1.4 on i686-pc-linux-gnu, compiled by GCC
i686-pc-linux-gnu-gcc (GCC) 3.4.6 (Gentoo 3.4.6-r1, ssp-3.4.5-1.0,
pie-8.7.9)"