Twilio sample test case generates an exception on Gather

I am running the test cases provided by Twilio: https://www.twilio.com/docs/voice/tutorials/ivr-phone-tree-csharp-mvc

All the tests of fail because the code throws an exception:

System.MissingMethodException HResult=0x80131513 Message=Method not found: 'Void Twilio.TwiML.Voice.Gather..ctor(System.Collections.Generic.List, System.Uri, Twilio.Http.HttpMethod, System.Nullable, System.String, System.Nullable, System.Nullable, System.String, System.Nullable, System.Uri, Twilio.Http.HttpMethod, LanguageEnum, System.String, System.Nullable, System.Nullable)

Here's an example of failing test case

    public void GivenAWelcomeAction_ThenTheResponseContainsGatherPlay()
    {
        var controllerPropertiesMock = new ControllerPropertiesMock();
        var controller = new IVRController
        {
            ControllerContext = controllerPropertiesMock.ControllerContext,
            Url = controllerPropertiesMock.Url(RouteConfig.RegisterRoutes)
        };

        controller.WithCallTo(c => c.Welcome())
            .ShouldReturnTwiMLResult(data =>
            {
                Assert.That(data.XPathSelectElement("Response/Gather/Say"), Is.Not.Null);
                Assert.That(data.XPathSelectElement("Response/Gather").Attribute("action").Value,
                    Is.EqualTo("/Menu/Show"));
            });
    }

Any suggestions as to how to avoid the exception?