diff --git a/letsencrypt/configuration.py b/letsencrypt/configuration.py
index a2eab5ecb73..f72005233f1 100644
--- a/letsencrypt/configuration.py
+++ b/letsencrypt/configuration.py
@@ -7,6 +7,7 @@
 from acme import challenges
 
 from letsencrypt import constants
+from letsencrypt import errors
 from letsencrypt import interfaces
 
 
@@ -36,11 +37,10 @@ class NamespaceConfig(object):
     def __init__(self, namespace):
         self.namespace = namespace
 
-        # XXX: breaks renewer in some bizarre way
-        #if self.simple_http_port == self.dvsni_port:
-        #    raise errors.Error(
-        #        "Trying to run SimpleHTTP non-TLS and DVSNI "
-        #        "on the same port ({0})".format(self.dvsni_port))
+        if self.simple_http_port == self.dvsni_port:
+            raise errors.Error(
+                "Trying to run SimpleHTTP and DVSNI "
+                "on the same port ({0})".format(self.dvsni_port))
 
     def __getattr__(self, name):
         return getattr(self.namespace, name)
diff --git a/letsencrypt/tests/configuration_test.py b/letsencrypt/tests/configuration_test.py
index c1eba85703d..44bccb57754 100644
--- a/letsencrypt/tests/configuration_test.py
+++ b/letsencrypt/tests/configuration_test.py
@@ -4,6 +4,8 @@
 
 import mock
 
+from letsencrypt import errors
+
 
 class NamespaceConfigTest(unittest.TestCase):
     """Tests for letsencrypt.configuration.NamespaceConfig."""
@@ -12,10 +14,15 @@ def setUp(self):
         self.namespace = mock.MagicMock(
             config_dir='/tmp/config', work_dir='/tmp/foo', foo='bar',
             server='https://acme-server.org:443/new',
-            dvsni_port='1234', simple_http_port=4321)
+            dvsni_port=1234, simple_http_port=4321)
         from letsencrypt.configuration import NamespaceConfig
         self.config = NamespaceConfig(self.namespace)
 
+    def test_init_same_ports(self):
+        self.namespace.dvsni_port = 4321
+        from letsencrypt.configuration import NamespaceConfig
+        self.assertRaises(errors.Error, NamespaceConfig, self.namespace)
+
     def test_proxy_getattr(self):
         self.assertEqual(self.config.foo, 'bar')
         self.assertEqual(self.config.work_dir, '/tmp/foo')