diff --git a/acme/acme/client_test.py b/acme/acme/client_test.py
index 5a43272f2..dee78910f 100644
--- a/acme/acme/client_test.py
+++ b/acme/acme/client_test.py
@@ -194,7 +194,7 @@ class ClientTest(unittest.TestCase):
         self.assertEqual(
             datetime.datetime(1999, 12, 31, 23, 59, 59),
             self.client.retry_after(response=self.response, default=10))
-        self.response.headers['Retry-After'] = 'Fri, 31 Dec 17 23:59:59 GMT'
+        self.response.headers['Retry-After'] = 'Fri, 31-Dec-17 23:59:59 GMT'
         self.assertEqual(
             datetime.datetime(2017, 12, 31, 23, 59, 59),
             self.client.retry_after(response=self.response, default=10))