diff --git a/html/ajax_setresolution.php b/html/ajax_setresolution.php
index b72bd748e..470b23da9 100644
--- a/html/ajax_setresolution.php
+++ b/html/ajax_setresolution.php
@@ -4,5 +4,6 @@ if(isset($_REQUEST['width']) AND isset($_REQUEST['height'])) {
     $_SESSION['screen_width'] = $_REQUEST['width'];
     $_SESSION['screen_height'] = $_REQUEST['height'];
 }
+header('Content-type: application/json');
 echo $_SESSION['screen_width'];
 echo $_SESSION['screen_height'];