diff --git a/phpBB/viewtopic.php b/phpBB/viewtopic.php
index e6708853c4..a0982ff07e 100644
--- a/phpBB/viewtopic.php
+++ b/phpBB/viewtopic.php
@@ -23,7 +23,10 @@
 include('extension.inc');
 include('common.'.$phpEx);
 
-$topic_id = $HTTP_GET_VARS[POST_TOPIC_URL];
+if(!isset($topic_id))
+{
+	$topic_id = $HTTP_GET_VARS[POST_TOPIC_URL];
+}
 
 $is_moderator = 0;