diff --git a/bumble/transport/serial.py b/bumble/transport/serial.py
index dca498fd..a812851b 100644
--- a/bumble/transport/serial.py
+++ b/bumble/transport/serial.py
@@ -17,6 +17,7 @@
 # -----------------------------------------------------------------------------
 import asyncio
 import logging
+from typing import Optional
 
 import serial_asyncio
 
@@ -51,7 +52,7 @@ class SerialPacketSource(StreamPacketSource):
         logger.debug('connection made')
         self._ready.set()
 
-    def connection_lost(self, exc: Exception | None) -> None:
+    def connection_lost(self, exc: Optional[Exception]) -> None:
         logger.debug('connection lost')
         self.on_transport_lost()