diff --git a/src/Eccube/Session/Storage/Handler/SameSiteNoneCompatSessionHandler.php b/src/Eccube/Session/Storage/Handler/SameSiteNoneCompatSessionHandler.php
index 89fad8a7c0c..31da74afdd5 100644
--- a/src/Eccube/Session/Storage/Handler/SameSiteNoneCompatSessionHandler.php
+++ b/src/Eccube/Session/Storage/Handler/SameSiteNoneCompatSessionHandler.php
@@ -207,6 +207,7 @@ public function getCookieSecure()
      */
     private function shouldSendSameSiteNone()
     {
-        return SameSite::handle($_SERVER['HTTP_USER_AGENT']);
+        $userAgent = array_key_exists('HTTP_USER_AGENT', $_SERVER) ? $_SERVER['HTTP_USER_AGENT'] : null;
+        return SameSite::handle($userAgent);
     }
 }