diff --git a/wchat/common/HttpClient.php b/wchat/common/HttpClient.php
index 97bb601..7f78d56 100644
--- a/wchat/common/HttpClient.php
+++ b/wchat/common/HttpClient.php
@@ -33,7 +33,7 @@ class HttpClient
 
 	/** @var string $_message 错误信息 */
 	private string $_message = '';
-	private string $_data = '';
+	private ?string $_data = '';
 
 	private int $connect_timeout = 1;