error null value in column" concat_id" violates not-null constraint

Hi,
I'm just beggining in PostgreSql 9.0

I've created a table ,like this:

CREATE TABLE auxiliar
(
ano integer,
codigodc character varying,
id character varying,
concat_id character varying NOT NULL,
CONSTRAINT concat PRIMARY KEY (concat_id);

Now i want to insert values on concat_id resulting from a function
called "concat_id"

INSERT INTO concat_id

SELECT(concat_id) FROM auxiliar ;

I get "error null value in column"concat_id" violatres not null
constraint

Could anyone tell me what am i doing wrong?

Ana