diff --git a/letsencrypt/plugins/standalone.py b/letsencrypt/plugins/standalone.py
index cb95ec408..e742734a9 100644
--- a/letsencrypt/plugins/standalone.py
+++ b/letsencrypt/plugins/standalone.py
@@ -182,11 +182,7 @@ class Authenticator(common.Plugin):
         return self.__doc__
 
     def prepare(self):  # pylint: disable=missing-docstring
-        if any(util.already_listening(port) for port in
-               (self.config.dvsni_port, self.config.simple_http_port)):
-            raise errors.MisconfigurationError(
-                "At least one of the (possibly) required ports is "
-                "already taken.")
+        pass
 
     def get_chall_pref(self, domain):
         # pylint: disable=unused-argument,missing-docstring
@@ -195,6 +191,12 @@ class Authenticator(common.Plugin):
         return chall_pref
 
     def perform(self, achalls):  # pylint: disable=missing-docstring
+        if any(util.already_listening(port) for port in
+               (self.config.dvsni_port, self.config.simple_http_port)):
+            raise errors.MisconfigurationError(
+                "At least one of the (possibly) required ports is "
+                "already taken.")
+
         try:
             return self.perform2(achalls)
         except errors.StandaloneBindError as error:
diff --git a/letsencrypt/plugins/standalone_test.py b/letsencrypt/plugins/standalone_test.py
index e99bd473a..b873da6f2 100644
--- a/letsencrypt/plugins/standalone_test.py
+++ b/letsencrypt/plugins/standalone_test.py
@@ -101,16 +101,16 @@ class AuthenticatorTest(unittest.TestCase):
     def test_more_info(self):
         self.assertTrue(isinstance(self.auth.more_info(), six.string_types))
 
-    @mock.patch("letsencrypt.plugins.standalone.util")
-    def test_prepare_misconfiguration(self, mock_util):
-        mock_util.already_listening.return_value = True
-        self.assertRaises(errors.MisconfigurationError, self.auth.prepare)
-        mock_util.already_listening.assert_called_once_with(1234)
-
     def test_get_chall_pref(self):
         self.assertEqual(set(self.auth.get_chall_pref(domain=None)),
                          set([challenges.DVSNI, challenges.SimpleHTTP]))
 
+    @mock.patch("letsencrypt.plugins.standalone.util")
+    def test_perform_misconfiguration(self, mock_util):
+        mock_util.already_listening.return_value = True
+        self.assertRaises(errors.MisconfigurationError, self.auth.perform, [])
+        mock_util.already_listening.assert_called_once_with(1234)
+
     @mock.patch("letsencrypt.plugins.standalone.zope.component.getUtility")
     def test_perform(self, unused_mock_get_utility):
         achalls = [1, 2, 3]