case bug?

Following case statement is legal but fails in 6.5.1.

drop table t1;
DROP
create table t1(i int);
CREATE
insert into t1 values(-1);
INSERT 4047465 1
insert into t1 values(0);
INSERT 4047466 1
insert into t1 values(1);
INSERT 4047467 1

select i,
case
when i < 0 then 'minus'
when i = 0 then 'zero'
when i > 0 then 'plus'
else null
end
from t1;
ERROR: Unable to locate type oid 0 in catalog

note that:

select i,
case
when i < 0 then 'minus'
when i = 0 then 'zero'
when i > 0 then 'plus'
end
from t1;

also causes the same error.
---
Tatsuo Ishii