notice after entering a function in template.php
That notification is displayed because the node key of the $vars array is not always defined in hook_preprocess_page() function.

Use the following code, which first checks if the node key exists and then continues. It also uses the switch statement, which is easier to read if you only need to check the node type and use one condition:

function myZen_preprocess_page(&amp;$vars) {
  if (isset($vars['node'])) {
    switch ($vars['node']-&gt;type) {
      case 'mappe':
        $vars['theme_hook_suggestions'][] = 'page__mappe';
        break;

      case 'fatti_e_risorse':
        $vars['theme_hook_suggestions'][] = 'page__fatti_e_risorse';
        break;
    }
  }
}