Is there a bug in FOR i IN 1..10 LOOP (8.4.4)?

CREATE FUNCTION test() RETURNS text
    LANGUAGE plpgsql
    AS $$DECLARE
  num_var TEXT;
BEGIN
  FOR i IN 1..10 LOOP
    num_var := num_var || ',' || i;
  END LOOP;
  RETURN num_var;
END;$$;

SELECT test();

returns NULL