diff --git a/web/index.php b/web/index.php
index 4a37241a7..7f823f1fd 100644
--- a/web/index.php
+++ b/web/index.php
@@ -55,7 +55,7 @@ require_once( 'includes/Monitor.php' );
 if (
   (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on')
   or
-  ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
+  (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) and ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https'))
 ) {
   $protocol = 'https';
 } else {