diff --git a/core/util.php b/core/util.php
index 8b20139d..80a86894 100644
--- a/core/util.php
+++ b/core/util.php
@@ -66,7 +66,7 @@ function contact_link(): ?string
 function is_https_enabled(): bool
 {
     // check forwarded protocol
-    if ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') {
+    if (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https') {
         $_SERVER['HTTPS']='on';
     }
     return (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off');