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.
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Tatsuo Ishii