diff --git a/testing/web-platform/tests/webdriver/tests/support/asserts.py b/testing/web-platform/tests/webdriver/tests/support/asserts.py
index a90934078fb483289e01c6bc8d7e5b0ffb06aedd..0d15964c9bba0a1e483b023f088a287de6bf7bc3 100644
--- a/testing/web-platform/tests/webdriver/tests/support/asserts.py
+++ b/testing/web-platform/tests/webdriver/tests/support/asserts.py
@@ -68,16 +68,18 @@ def assert_error(response, error_code):
     assert isinstance(response.body["value"]["message"], basestring)
     assert isinstance(response.body["value"]["stacktrace"], basestring)
 
-def assert_success(response, value):
+def assert_success(response, value=None):
     """Verify that the provided wdclient.Response instance described a valid
     error response as defined by `dfn-send-an-error` and the provided error
     code.
-    :param response: wdclient.Response instance
-    :param value: expected value of the response body
-    """
 
+    :param response: wdclient.Response instance.
+    :param value: Expected value of the response body, if any.
+
+    """
     assert response.status == 200
-    assert response.body["value"] == value
+    if value is not None:
+        assert response.body["value"] == value
 
 def assert_dialog_handled(session, expected_text):
     result = session.transport.send("GET",