diff --git a/libraries/header.inc.php b/libraries/header.inc.php
index dee9b1532..a4fdfc687 100644
--- a/libraries/header.inc.php
+++ b/libraries/header.inc.php
@@ -17,7 +17,7 @@ require_once './libraries/common.inc.php';
 /**
  * If this is an Ajax request, we do not need to generate all this output.
  */
-if (!$GLOBALS['is_ajax_request']) {
+if (isset($GLOBALS['is_ajax_request']) && !$GLOBALS['is_ajax_request']) {
 
     if (empty($GLOBALS['is_header_sent'])) {