diff --git a/src/paho/mqtt/client.py b/src/paho/mqtt/client.py
index 347d8ad2..754124bf 100644
--- a/src/paho/mqtt/client.py
+++ b/src/paho/mqtt/client.py
@@ -728,7 +728,7 @@ def on_connect(client, userdata, flags, reason_code, properties):
 
     def __init__(
         self,
-        callback_api_version: CallbackAPIVersion,
+        callback_api_version: CallbackAPIVersion = CallbackAPIVersion.VERSION1,
         client_id: str | None = "",
         clean_session: bool | None = None,
         userdata: Any = None,