Possible bug in referential integrity system

The submit button on the form located at
http://www.postgresql.org/bugs/bugs.php?1 results in a report of a
parsing error, so I'm sending this here as the alternative.

Is the following a bug in the referential integrity system?  This is
for PG 7.0.0.  I realize that declaring the "ref" column in t2
additionally as "not null" would prevent this.  However, why does the
"references" check allow insertion of a null value into t2 when there
are no corresponding null value in the num column of t1?

If this is actually fixed in 7.0.2, then please accept my apologies.

===============

=> create table t1 (num int4, name text);
CREATE
=> create table t2 (ref int4 references t1 (num), val text);
NOTICE:  CREATE TABLE will create implicit trigger(s) for FOREIGN KEY check(s)
CREATE

=> insert into t1 values (1, 'Widget1');
INSERT 17518650 1

=> insert into t2 values ( (select num from t1 where name = 'Widget1'), 'Valuable');
INSERT 17518651 1
=> insert into t2 values ( (select num from t1 where name = 'widget2'), 'Bug?');
INSERT 17518652 1

=> select * from t2;
 ref |   val
-----+----------
   1 | Valuable
     | Bug?
(2 rows)

=> select * from t2 where ref is null;
 ref | val
-----+------
     | Bug?
(1 row)

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