diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0712696..4405439 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [unreleased]
 
+- fix exception in MODBUS timeout callback
+
 ## [0.9.0-RC1] - 2024-06-29
 
 - add asyncio log and debug mode
diff --git a/app/src/modbus.py b/app/src/modbus.py
index 57eb272..7425b56 100644
--- a/app/src/modbus.py
+++ b/app/src/modbus.py
@@ -108,6 +108,7 @@ class Modbus():
     def close(self):
         """free the queue and erase the callback handlers"""
         logging.debug('Modbus close:')
+        self.__stop_timer()
         self.rsp_handler = None
         self.snd_handler = None
         while not self.que.empty:
@@ -252,6 +253,7 @@ class Modbus():
         # logging.debug(f'Modbus stop timer {self}')
         if self.tim:
             self.tim.cancel()
+            self.tim = None
 
     def __timeout_cb(self) -> None:
         '''Rsponse timeout handler retransmit pdu or send next pdu'''