cache lookup failed for type 0

Hi all,
doing some tests:

regression=# create table test (a integer);
CREATE TABLE
regression=# alter table test add column b test;
ALTER TABLE
regression=# select * from test;
a | b
---+---
(0 rows)

regression=# insert into test values ( 1, (2,null)); <--(1)
INSERT 4260928 1

regression=# \d test
Table "public.test"
Column | Type | Modifiers
--------+---------+-----------
a | integer |
b | test |

regression=# alter table test drop column a;
ALTER TABLE
regression=# insert into test values ( (3,null));
ERROR: cache lookup failed for type 0

regression=# select * from test;
b
----
()
(1 row)

regression=# alter table test add column c integer;
ALTER TABLE
kalman=# select * from test;
b | c
-----+---
(,) |
(1 row)

regression=# insert into test values ( (null,3), 4); <--(2)
ERROR: cannot cast type record to test
DETAIL: Input has wrong number of columns.

regression=# \d test
Table "public.test"
Column | Type | Modifiers
--------+---------+-----------
b | test |
c | integer |

The two insert in (1) and (2) are the same ( change only the column order ),
but the (1) works and the (2) not.

Regards
Gaetano Mendola