Problema ao recuperar elemento usando Jsoup

try {
        Connection conn = Jsoup.connect("http://denatran.serpro.gov.br/certificado/veiculo.asp");
        Document d = conn.get();
        Element captcha = d.getElementById("img_captcha_serpro_gov_br");
        if (captcha == null) {
            throw new RuntimeException("Unable to find captcha...");
        }

        Connection.Response response = Jsoup //
                .connect(captcha.absUrl("src")) // Extract image absolute URL
                .cookies(conn.response().cookies()) // Grab cookies
                .ignoreContentType(true) // Needed for fetching image
                .execute();

        ImageIcon image = new ImageIcon(ImageIO.read(new ByteArrayInputStream(response.bodyAsBytes())));
       
        JOptionPane.showMessageDialog(null, image, "Captcha image", JOptionPane.PLAIN_MESSAGE);
    } catch (IOException ex) {
        System.err.println(ex);
    }