diff --git a/lib/activityobject.php b/lib/activityobject.php
index 61614935f1..5185d77610 100644
--- a/lib/activityobject.php
+++ b/lib/activityobject.php
@@ -105,6 +105,7 @@ class ActivityObject
     public $thumbnail;
     public $largerImage;
     public $description;
+    public $extra = array();
 
     /**
      * Constructor