diff --git a/library/think/Request.php b/library/think/Request.php
index 4edf5083..d565eeb0 100644
--- a/library/think/Request.php
+++ b/library/think/Request.php
@@ -1228,7 +1228,7 @@ class Request
     {
         if (isset($_SERVER['HTTP_VIA']) && stristr($_SERVER['HTTP_VIA'], "wap")) {
             return true;
-        } elseif (strpos(strtoupper($_SERVER['HTTP_ACCEPT']), "VND.WAP.WML")) {
+        } elseif (isset($_SERVER['HTTP_ACCEPT']) && strpos(strtoupper($_SERVER['HTTP_ACCEPT']), "VND.WAP.WML")) {
             return true;
         } elseif (isset($_SERVER['HTTP_X_WAP_PROFILE']) || isset($_SERVER['HTTP_PROFILE'])) {
             return true;