problem with plpgsql function

Hi,

I have a table "isbn" (postgres-7.0.2) with two columns:

book_id int4
isbn varchar(20)

I want to create a function that joins all isbn for specified book_id.
The code of this function is:

CREATE FUNCTION all_isbn2( int4 ) RETURNS varchar AS 'DECLARE
i isbn%ROWTYPE;
res varchar;
BEGIN res := "";
FOR i IN SELECT * FROM isbn WHERE isbn.book_id = $1 LOOP
res := res || i.isbn;
END LOOP;
RETURN res;
END;' LANGUAGE 'plpgsql';

But when i execute the query

book=> select all_isbn2(1);

I receive the following error:

ERROR: Attribute '' not found

What I'm doing wrong?

Regards,
Alex