--- includes/rules.state.inc	2012-08-03 02:48:35.000000000 -0500
+++ includes/rules.state.inc	2012-12-17 14:53:20.000000000 -0600
@@ -617,7 +617,9 @@
    */
   public static function addSiteMetadata(EntityMetadataWrapper $wrapper, $property_info) {
     $site_info = entity_get_property_info('site');
-    $property_info['properties'] += $site_info['properties'];
+    if (isset($site_info['properties'])) {
+      $property_info['properties'] += $site_info['properties'];
+    }
     // Also invoke the usual callback for altering metadata, in case actions
     // have specified further metadata.
     return RulesData::applyMetadataAssertions($wrapper, $property_info);