diff --git a/async_substrate_interface/async_substrate.py b/async_substrate_interface/async_substrate.py
index a93af0c..0fe3969 100644
--- a/async_substrate_interface/async_substrate.py
+++ b/async_substrate_interface/async_substrate.py
@@ -874,7 +874,7 @@ def __init__(
         retry_timeout: float = 60.0,
         _mock: bool = False,
         _log_raw_websockets: bool = False,
-        ws_shutdown_timer: float = 5.0,
+        ws_shutdown_timer: Optional[float] = 5.0,
         decode_ss58: bool = False,
     ):
         """