diff --git a/src/pyob/entrance_mixins.py b/src/pyob/entrance_mixins.py
index c74bab7..d411f71 100644
--- a/src/pyob/entrance_mixins.py
+++ b/src/pyob/entrance_mixins.py
@@ -152,7 +152,7 @@ def _dynamic_do_POST_method(handler_instance: ObserverHandler) -> None:
 
         ObserverHandler.controller = self
 
-        def run_server():
+        def run_server() -> None:
             try:
                 server = HTTPServer(("localhost", 5000), ObserverHandler)
                 server.serve_forever()