How to use CAPTCHA with TLS

I am trying to use CAPTCHA in registration web page with TLS. I installed ejabberd 2.1.6 and configured as

{hosts, ["example.org"]}.

{captcha_cmd, ...}.
{captcha_host, "example.org:5281"}.

{listen,
 [
  ...
  {5281, ejabberd_http, [
                         tls,
                         {certfile, ...},
                         captcha,
                         register
                        ]}
 ]}.

{modules,
 [
  ...
  {mod_register_web, []},
  ...
 ]}.

But, CAPTCHA image is not shown in registration page https://example.org:5281/register/new. I wonder URL for the image is "http://", not "https://" in HTML source of the page.

Please tell me how to use CAPTCHA with TLS.