diff --git a/wchat/common/HttpClient.php b/wchat/common/HttpClient.php
index 6a51722..1034303 100644
--- a/wchat/common/HttpClient.php
+++ b/wchat/common/HttpClient.php
@@ -21,7 +21,7 @@ class HttpClient
 	private $method = 'get';
 
 	private $isSSL = false;
-	private $agent = '';
+	private $agent = null;
 	private $errorCodeField = '';
 	private $errorMsgField = '';
 	private $use_swoole = false;