diff --git a/Lib/pydoc.py b/Lib/pydoc.py
index 39db3915dc2cac9..892cdb49f6ffa39 100644
--- a/Lib/pydoc.py
+++ b/Lib/pydoc.py
@@ -2266,6 +2266,9 @@ def ready(self, server):
         def stop(self):
             """Stop the server and this thread nicely"""
             self.docserver.quit = True
+            # explicitly break a reference cycle: DocServer.callback
+            # has indirectly a reference to ServerThread.
+            self.docserver = None
             self.serving = False
             self.url = None