code in docs gives me an error

Hi
I tried creating the function (from sec 19.3.1). But it gives me the
following error: "ERROR: type tablename does not exist".
Any ideas of what i might be doing wrong?

CREATE FUNCTION use_many_fields(tablename) RETURNS TEXT AS '
DECLARE
in_t ALIAS FOR $1;
BEGIN
RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
' LANGUAGE 'plpgsql';

Thanks,
--Hicham