diff --git a/python_apps/media-monitor2/media/monitor/eventcontractor.py b/python_apps/media-monitor2/media/monitor/eventcontractor.py
index 7fa13b8cc..f9ff96dd7 100644
--- a/python_apps/media-monitor2/media/monitor/eventcontractor.py
+++ b/python_apps/media-monitor2/media/monitor/eventcontractor.py
@@ -48,6 +48,8 @@ class EventContractor(Loggable):
             # checked against the newest event 'evt' in this case
             self.unregister( old_e )
         evt.add_safe_pack_hook( lambda : self.__unregister(evt) )
+        assert evt.path not in self.store, \
+            "Clean up should have been called by '%s'" % evt
         self.store[ evt.path ] = evt
         return True # We actually added something, hence we return true.