diff --git a/canopen/sync.py b/canopen/sync.py
index dcef5ebc..3619cfff 100644
--- a/canopen/sync.py
+++ b/canopen/sync.py
@@ -36,4 +36,5 @@ def start(self, period=None):
 
     def stop(self):
         """Stop periodic transmission of SYNC message."""
-        self._task.stop()
+        if self._task is not None:
+            self._task.stop()