protocol variable missed in static list

In custom_url_rewrite() various static variables are seeded on the first call to the function:

    if (is_null($default)) {
      // initialise defaults
      $default = variable_get('singlesignon_master_url','');
      $force_default = variable_get('multidomain_force_default_domain', false);
      $domains = variable_get('multidomain_sites', array());
      $protocol = $_SERVER['HTTPS'] ? 'https://' : 'http://' ;
    }

$protocol was forgotten from the list of static variables, so on subsequent calls it's presumably null. Need to set it as a static variable.