6.5.1 -vs- null values for an int8

Any clue why Postgresql version 6.5.1 can't handle null values for int8 in
a WHERE clause?

incanta=> create table t(v int8);
CREATE
incanta=> insert into t(v) values(0);
INSERT 101737 1
incanta=> insert into t(v) values(1);
INSERT 101738 1
incanta=> insert into t(v) values(-1);
INSERT 101739 1
incanta=> select * from t where v>=0;
v
-
0
1
(2 rows)

incanta=> insert into t(v) values (null);
INSERT 101740 1
incanta=> select * from t;
v
--
0
1
-1

(4 rows)

incanta=> select * from t where v>=0;
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
We have lost the connection to the backend, so further processing is impossible. Terminating.