[RESOLVED] URGENT ERROR - CUSTOMERS UNABLE TO REGISTER

I have done a completely clean installation of OpenCart 1.5.3.1 at http://estore.z01.co.uk. Now everytime a customer attempts to register at the site we get this error:

Notice: Undefined index: zone_id in /home/---/public_html/subDomain/catalog/controller/account/register.php on line 423

Does anyone have a solution for this?

I have manage to resolve the issue myself. It turns out the template from ThemeForest I had was missing an update for the new OC 1.5.3.1. I've now got the latest version and it now works properly.