diff --git a/common.php b/common.php
index 14178c8..80cefd7 100644
--- a/common.php
+++ b/common.php
@@ -1,8 +1,9 @@
 <?php
 function get_protocol() {
 	$protocol = "http";
-	if ( (!empty($_SERVER['HTTPS'] && $_SERVER['HTTPS'] !== 'off') ||
-		$_SERVER['SERVER_PORT'] == 443) )
+	if ( (isset($_SERVER['HTTPS']) && !empty($_SERVER['HTTPS']
+			&& $_SERVER['HTTPS'] !== 'off') ||
+			$_SERVER['SERVER_PORT'] == 443) )
 		$protocol = "https";
 	return $protocol."://";
 }