diff --git a/Lib/distutils/extension.py b/Lib/distutils/extension.py
index 94b56e6..e69f3e9 100644
--- a/Lib/distutils/extension.py
+++ b/Lib/distutils/extension.py
@@ -82,6 +82,8 @@
         from the source extensions if not provided.
     """
 
+    # When adding arguments to this constructor, be sure to update
+    # setup_keywords in core.py.
     def __init__ (self, name, sources,
                   include_dirs=None,
                   define_macros=None,