insert inserted id to another table

Here's the scenario:

create table a (
 id serial primary key,
 val text
);

create table b (
 id serial primary key,
 a_id integer references a(id)
);

create rule a_inserted as on insert to a do also insert into b (a_id) values (new.id);

I'm trying to create a record in b referencing to a on insertion to a table, but what I get is that new.id is null, as it's automatically generated from sequence. I also tried it with trigger, that is launched AFTER insert FOR EACH ROW on a table, but result was the same. Any way to work this out?