warning displayed on the Zendesk settings page within drupal (after upgrading to 6.x-1.3)

I just upgraded to zendesk 6.x-1.3 (I guess from 6.x-1.x) and now when I go to my Administer > Site Configuration > Zendesk Settings page I get a warning at the top saying:

warning: Invalid argument supplied for foreach() in C:\HostingSpaces\priory01\support.priorysolutions.com\wwwroot\includes\form.inc on line 1205.

I'm a newbie with drupal so it might be to do with how I upgraded:
- disabled the zendesk module
- deleted the zendesk folder sites/all/modules/zendesk
- uploaded the new zendesk folder
- went to update.php
- realised I hadn't enabled the zendesk module, so did that
- went to update.php

That line 1205 corresponds to this:

function form_type_checkboxes_value($form, $edit = FALSE) {
  if ($edit === FALSE) {
    $value = array();
    $form += array('#default_value' => array());
THE BELOW LINE IS 1205: 
    foreach ($form['#default_value'] as $key) {
      $value[$key] = 1;
    }
    return $value;
  }
  elseif (!isset($edit)) {
    return array();
  }
}