diff --git a/src/pyg2o/server.py b/src/pyg2o/server.py
index 300ea9f..0bc0d32 100644
--- a/src/pyg2o/server.py
+++ b/src/pyg2o/server.py
@@ -75,7 +75,7 @@ class Server:
     @classmethod
     def _register_routes(cls, app):
         @app.websocket('/pyg2o')
-        async def pyg2o(websocket: WebSocket, token: str, topics: str | None):
+        async def pyg2o(websocket: WebSocket, token: str, topics: str | None = None):
             await cls._handle_connection(websocket, token, topics)
 
         _ = pyg2o