Mocking a ViewModel for unit testing with Moq?

New to unit testing. I have a WPF client app hooked into a WCF service via basicHttpbinding. Everything works great. I'm using simple constructor Dependency Injection in my viewModel, passing in an IServiceChannel which I then call me service methods on e.g:

IMyserviceChannel = MyService;

public MyViewModel(IMyServiceChannel myService
{
   this.MyService = myService;  
}

Private void GetPerson()
{
  var selectedPerson = MyService.GetSelectedPerson();
}

I have then added an MS Test project in the client app and I'm trying to use Moq to mock my service:

  [TestMethod]
    public void GetArticleBody_Test_Valid()
    {
        // Create channel mock
        Mock<IIsesServiceChannel> channelMock = new Mock<IIsesServiceChannel>(MockBehavior.Strict);    

        // setup the mock to expect the Reverse method to be called
        channelMock.Setup(c => c.GetArticleBody(1010000008)).Returns("110,956 bo/d, 1.42 Bcfg/d and 4,900 bc/d. ");

        // create string helper and invoke the Reverse method
        ArticleDataGridViewModel articleDataGridViewModel = new ArticleDataGridViewModel(channelMock.Object);
        string result = channelMock.GetArticleBody(1010000008);
        //Assert.AreEqual("cba", result);

        //verify that the method was called on the mock
        channelMock.Verify(c => c.GetArticleBody(1010000008), Times.Once());
    }

The test is failing with a System.NullReferenceException but I don't know why this is?