--- a/svghmi/svghmi_server.py	Tue Feb 11 13:56:48 2020 +0100
+++ b/svghmi/svghmi_server.py	Thu Feb 13 09:41:43 2020 +0100
@@ -92,6 +92,8 @@
         if duration:
             self.timer = Timer(duration, self.trigger)
             self.timer.start()
+        else:
+            self.timer = None
 
     def _stop(self):
         if self.timer is not None: