diff --git a/install/index.php b/install/index.php
index f1547982c..350c2f2d0 100755
--- a/install/index.php
+++ b/install/index.php
@@ -139,7 +139,7 @@
 	}
 
 	function is_server_https() {
-		return (!empty($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] != 'off')) || $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https';
+		return (!empty($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] != 'off')) || (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https');
 	}
 
 	function make_self_url_path() {