Index: Lib/urllib2.py
===================================================================
--- Lib/urllib2.py	(revision 56224)
+++ Lib/urllib2.py	(working copy)
@@ -339,6 +339,8 @@
             bisect.insort(self.handlers, handler)
             handler.add_parent(self)
 
+        return added
+
     def close(self):
         # Only exists for backwards compatibility.
         pass
@@ -459,7 +461,8 @@
     for h in handlers:
         if isclass(h):
             h = h()
-        opener.add_handler(h)
+        if not opener.add_handler(h):
+            raise ValueError, "%s is not a handler" % type(h)
     return opener
 
 class BaseHandler:
Index: Lib/test/test_urllib2.py
===================================================================
--- Lib/test/test_urllib2.py	(revision 56224)
+++ Lib/test/test_urllib2.py	(working copy)
@@ -1019,6 +1019,8 @@
             def foo_open(self): pass
         class BarHandler(urllib2.BaseHandler):
             def bar_open(self): pass
+        class NotActuallyAHandler:
+            pass
 
         build_opener = urllib2.build_opener
 
@@ -1044,6 +1046,8 @@
         o = build_opener(urllib2.HTTPHandler())
         self.opener_has_handler(o, urllib2.HTTPHandler)
 
+        self.assertRaises(ValueError, build_opener, NotActuallyAHandler)
+
     def opener_has_handler(self, opener, handler_class):
         for h in opener.handlers:
             if h.__class__ == handler_class: