diff --git a/acme/acme/client_test.py b/acme/acme/client_test.py
index 3aac9c874..4bd762865 100644
--- a/acme/acme/client_test.py
+++ b/acme/acme/client_test.py
@@ -181,7 +181,7 @@ class ClientTest(unittest.TestCase):
 
         # TODO: split here and separate test
         self.assertRaises(errors.UnexpectedUpdate, self.client.answer_challenge,
-                          self.challr.body.update(_uri='foo'), chall_response)
+                          self.challr.body.update(uri='foo'), chall_response)
 
     def test_answer_challenge_missing_next(self):
         self.assertRaises(
diff --git a/acme/acme/messages.py b/acme/acme/messages.py
index 4dee96c58..2ac29941e 100644
--- a/acme/acme/messages.py
+++ b/acme/acme/messages.py
@@ -326,8 +326,9 @@ class ChallengeBody(ResourceBody):
     """
     __slots__ = ('chall',)
     # ACMEv1 has a "uri" field in challenges. ACMEv2 has a "url" field. This
-    # challenge object supports either one. In Client.answer_challenge,
-    # whichever one is set will be used.
+    # challenge object supports either one, but should be accessed through the
+    # name "uri". In Client.answer_challenge, whichever one is set will be
+    # used.
     _uri = jose.Field('uri', omitempty=True, default=None)
     _url = jose.Field('url', omitempty=True, default=None)
     status = jose.Field('status', decoder=Status.from_json,
@@ -337,13 +338,12 @@ class ChallengeBody(ResourceBody):
                        omitempty=True, default=None)
 
     def __init__(self, **kwargs):
-        new_kwargs = {}
-        for k, v in kwargs.items():
-            if k in ('uri', 'url',):
-                k = '_' + k
-            new_kwargs[k] = v
+        kwargs = dict((self._internal_name(k), v) for k, v in kwargs.items())
         # pylint: disable=star-args
-        super(ChallengeBody, self).__init__(**new_kwargs)
+        super(ChallengeBody, self).__init__(**kwargs)
+
+    def encode(self, name):
+        return super(ChallengeBody, self).encode(self._internal_name(name))
 
     def to_partial_json(self):
         jobj = super(ChallengeBody, self).to_partial_json()
@@ -364,6 +364,15 @@ class ChallengeBody(ResourceBody):
     def __getattr__(self, name):
         return getattr(self.chall, name)
 
+    def __iter__(self):
+        # When iterating over fields, use the external name 'uri' instead of
+        # the internal '_uri'.
+        for name in super(ChallengeBody, self).__iter__():
+            yield name[1:] if name == '_uri' else name
+
+    def _internal_name(self, name):
+        return '_' + name if name == 'uri' else name
+
 
 class ChallengeResource(Resource):
     """Challenge Resource.
diff --git a/acme/acme/messages_test.py b/acme/acme/messages_test.py
index 631f0ce4d..c9e5c2cf1 100644
--- a/acme/acme/messages_test.py
+++ b/acme/acme/messages_test.py
@@ -283,6 +283,9 @@ class ChallengeBodyTest(unittest.TestCase):
             'detail': 'Unable to communicate with DNS server',
         }
 
+    def test_encode(self):
+        self.assertEqual(self.challb.encode('uri'), self.challb.uri)
+
     def test_to_partial_json(self):
         self.assertEqual(self.jobj_to, self.challb.to_partial_json())