diff --git a/cherrypy/_cpengine.py b/cherrypy/_cpengine.py
index <HASH>..<HASH> 100644
--- a/cherrypy/_cpengine.py
+++ b/cherrypy/_cpengine.py
@@ -59,6 +59,8 @@ class Engine(object):
         
         self.mtimes = {}
         self.reload_files = []
+        
+        self.monitor_thread = None
     
     def start(self, blocking=True):
         """Start the application engine."""
Engine.monitor_thread defaults to None now.

This is in case an Exception is raised in an on_start_engine function and the code to start/stop the engine is wrapped in a try/finally, where Engine.stop is called in finally.