diff --git a/library/think/Lang.php b/library/think/Lang.php
index 23887a2c..9e6adb3f 100644
--- a/library/think/Lang.php
+++ b/library/think/Lang.php
@@ -201,7 +201,7 @@ class Lang
         } elseif (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
             // 自动侦测浏览器语言
             preg_match('/^([a-z\d\-]+)/i', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches);
-            $langSet     = strtolower($matches[1]);
+            $langSet     = strtolower($matches[1] ?? '');
             $acceptLangs = Config::get('header_accept_lang');
 
             if (isset($acceptLangs[$langSet])) {