diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index fdf2b67..0df66ad 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -4453,6 +4453,12 @@
         self.assertTrue(ns2 != ns3)
         self.assertTrue(ns2 != ns4)
 
+    def test_equality_returns_notimplemeted(self):
+        # See issue 21481
+        ns = argparse.Namespace(a=1, b=2)
+        self.assertIs(ns.__eq__(None), NotImplemented)
+        self.assertIs(ns.__ne__(None), NotImplemented)
+
 
 # ===================
 # File encoding tests