diff --git a/tinyfilemanager.php b/tinyfilemanager.php
index 510c6a4..3b111b4 100644
--- a/tinyfilemanager.php
+++ b/tinyfilemanager.php
@@ -257,7 +257,7 @@ if (empty($auth_users)) {
     $use_auth = false;
 }
 
-$is_https = isset($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] == 'on' || $_SERVER['HTTPS'] == 1)
+$is_https = isset($_SERVER['HTTPS']) && (strtolower($_SERVER['HTTPS']) == 'on' || $_SERVER['HTTPS'] == 1)
     || isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https';
 
 // update $root_url based on user specific directories