diff --git a/demo/worker/worker_web_service.py b/demo/worker/worker_web_service.py
index 203527229..832bceccd 100644
--- a/demo/worker/worker_web_service.py
+++ b/demo/worker/worker_web_service.py
@@ -103,7 +103,7 @@ class WorkerWebService(WebService):
         '''
         Stops the worker run.
         '''
-        self.params_data = None
+        self.params_data = {}
         self.is_simulation_running = False
         return ('', 204)