Can't join on null values

PostgreSQL version: 7.4.3 (RPMs from ftp.au.postgresql.org)

Operating Sysem: Fedora Core 1

CREATE TABLE t1 (i INTEGER, j INTEGER);
INSERT INTO t1 VALUES (1, NULL);
CREATE TABLE t2 AS SELECT * FROM t1;
SELECT * FROM t1 JOIN t2 USING (i, j);
i | j
---+---
(0 rows)

I believe the one row, which is identically present in both table,
should be selected. The problem occurs because of the NULL value.