Problem with recaptcha plugin. Help me!!!

I have a problem with recaptcha plugin in my joomla 1.5.22 site. I install e activated the plugin but when I go on register form it appears this fatal error:

Fatal error: Call to undefined function hikashop_isSSL() in /xxx/xxx/xxxxxxx/plugins/system/recaptcha.php on line 96

Any solution?
HELP ME!!!
PLEASE

p.s.
I have some custom field activated in the registration form.

Can you check in your plugin if the line:

if(!include_once(rtrim(JPATH_ADMINISTRATOR,DS).DS.'components'.DS.'com_hikashop'.DS.'helpers'.DS.'helper.php')) return true;

Yes it is present in line 81 exactly!
It seems that the problem is with the custom field of hikashop because when I disable them it works...

I solved the problem!!!!!

I change the code:
recaptcha_get_html($this->params->get('public_key'),null,hikashop_isSSL())

with:
recaptcha_get_html($this->params->get('public_key'))


in the file plugins/system/recaptcha.php