Referential integrity and inheritance.

Please let me know what I've missed about the referential
integrity ?
(using PG v.7.0.2)

test=> create table objects (id serial);
NOTICE: CREATE TABLE will create implicit sequence 'objects_id_seq' for SERIAL column 'objects.id'
NOTICE: CREATE TABLE/UNIQUE will create implicit index 'objects_id_key' for table 'objects'
CREATE

test=> create table books (title name) inherits(objects);
CREATE

test=> create table opinions (idobject int4 REFERENCES objects(id) ON UPDATE CASCADE, comment text);
NOTICE: CREATE TABLE will create implicit trigger(s) for FOREIGN KEY check(s)
NOTICE: _outNode: don't know how to print type 726
CREATE

test=> insert into books (title) values ('DataBases');
INSERT 691707 1
test=> select * from objects*;
id
----
1
(1 row)

test=> select * from books;
id | title
----+-----------
1 | DataBases
(1 row)

test=> insert into opinions (idobject, comment) values (1, 'Very Wise');
ERROR: <unnamed> referential integrity violation - key referenced from opinions not found in objects

test=>