Inheritance

Hi, I'm new on postgres and I've just installed postgres 7.4.7 on a
debian sarge. I'm interested on using inheritance. I've tried a simple code:

CREATE TABLE t_main (
id serial primary key
);

CREATE TABLE t_derived1 (
field1 varchar(128) default NULL
) INHERITS (t_main);

Now I have to create another table having a field having a reference to
t_derived1. If I use the code
CREATE TABLE t_table1 (
id serial primary key,
id_derived1 int references t_derived1
);
I got an error: t_derived1 have no primary key... and in effect is
t_main that have the primary key... So I modified the code in
CREATE TABLE t_table1 (
id serial primary key,
id_derived1 int references t_main
);
and now all seems to work so I inserted a record on t_derived1

INSERT INTO t_derived1
(field1) VALUES ('field1 content of derived1 table');

and a record in t_table1 that have a reference to the record I've just
inserted:

INSERT INTO t_table1
(id_derived1) VALUES (1);

but I've got the error 'ERROR: insert or update on table "t_table1"
violates foreign key constraint "$1"
DETAIL: Key (id_derived1)=(1) is not present in table "t_main".'

So I ask you: there is a way to reference a record to an hinherited table?

Thank you very much

Piviul