Index: urllib2.py
===================================================================
--- urllib2.py	(revision 56286)
+++ urllib2.py	(working copy)
@@ -295,6 +295,9 @@
         self.process_request = {}
 
     def add_handler(self, handler):
+        if not hasattr(handler, "add_parent"):
+            raise TypeError("expected BaseHandler instance, got %r" % handler)
+
         added = False
         for meth in dir(handler):
             if meth in ["redirect_request", "do_open", "proxy_open"]:
Index: test/test_urllib2.py
===================================================================
--- test/test_urllib2.py	(revision 56286)
+++ test/test_urllib2.py	(working copy)
@@ -381,6 +381,12 @@
 
 class OpenerDirectorTests(unittest.TestCase):
 
+    def test_add_non_handler(self):
+        class NonHandler(object):
+            pass
+        self.assertRaises(TypeError,
+                          OpenerDirector().add_handler, NonHandler())
+
     def test_badly_named_methods(self):
         # test work-around for three methods that accidentally follow the
         # naming conventions for handler methods