Fix WebSocketClientConnection parameter

diff --git a/tornado/websocket.py b/tornado/websocket.py
index 165cc316d601625ab657c6be7bb6c8ff94d507ce..fbfd70088772ed89b5b7f2a582e45a36d8ff0c6e 100644 (file)
--- a/tornado/websocket.py
+++ b/tornado/websocket.py
@@ -1357,7 +1357,7 @@ class WebSocketClientConnection(simple_httpclient._HTTPConnection):
         ping_interval: Optional[float] = None,
         ping_timeout: Optional[float] = None,
         max_message_size: int = _default_max_message_size,
-        subprotocols: Optional[List[str]] = [],
+        subprotocols: Optional[List[str]] = None,
         resolver: Optional[Resolver] = None,
     ) -> None:
         self.connect_future = Future()  # type: Future[WebSocketClientConnection]