diff --git a/include/class.thread.php b/include/class.thread.php
index b1a0f1bf7a0eb462bf5df7acb3ff0eae4a3321ad..0b2df9263ca3e1c92380ce1edc71ccb60bd85e9b 100644
--- a/include/class.thread.php
+++ b/include/class.thread.php
@@ -2863,7 +2863,8 @@ implements TemplateVariable {
     function addResponse($vars, &$errors) {
         $vars['threadId'] = $this->getId();
         $vars['userId'] = 0;
-        $vars['pid'] = $this->getLastMessage()->getId();
+        if ($message = $this->getLastMessage())
+            $vars['pid'] = $message->getId();
 
         $vars['flags'] = 0;
         switch ($vars['reply-to']) {