diff --git a/Lib/idlelib/WindowList.py b/Lib/idlelib/WindowList.py
index 6de3e5855c5..7e05a57c032 100644
--- a/Lib/idlelib/WindowList.py
+++ b/Lib/idlelib/WindowList.py
@@ -39,7 +39,7 @@ def register_callback(self, callback):
 
     def unregister_callback(self, callback):
         try:
-           self.callbacks.remove(callback)
+            self.callbacks.remove(callback)
         except ValueError:
             pass