Problems w. SERIAL

Hi.

Have the following tables:

CREATE TABLE tbl_a (id SERIAL PRIMARY KEY, data VARCHAR(5));
CREATE TABLE tbl_b (id SERIAL PRIMARY KEY, data VARCHAR(5));

CREATE TABLE tbl_c
 (id SERIAL PRIMARY KEY,
  data VARCHAR(50),
  a SERIAL CONSTRAINT a_ref REFERENCES tbl_a(id),
  b SERIAL CONSTRAINT b_ref REFERENCES tbl_b(id)
);

The two inserts into tbl_a and tbl_b work fine:

INSERT INTO tbl_a (data) VALUES ('a data');
INSERT INTO tbl_b (data) VALUES ('b data');

But if I do

INSERT INTO tbl_c (data, a, b) VALUES ('c data',1,1);

I get ERROR:  Relation 'tbl_c_id_seq' does not exist
even though that sequence was implicitly created upon
creation of tbl_c - or?

Can anyone help me out on this? Thanks.

Morten