单元测试FindAsync

我使用这篇优秀的MSDN文章中的异步查询提供程序设置了一个测试项目:http://msdn.microsoft.com/en-US/data/dn314429#async,效果很好.

但是当我添加一个调用的方法时FindAsync:

public async Task GetBlog(int blogId)
{
    return await _context.Blogs.FindAsync(blogId);
}

并以以下格式添加以下单元测试:

[TestMethod]
public async Task GetAllBlogsAsync_gets_blog()
{
    var data = new List
    {
        new Blog { BlogId = 1, Name = "BBB" },
        new Blog { BlogId = 2, Name = "ZZZ" },
        new Blog { BlogId = 3, Name = "AAA" },
    }.AsQueryable();

    var mockSet = new Mock>();
    mockSet.As>()
        .Setup(m => m.GetAsyncEnumerator())
        .Returns(new TestDbAsyncEnumerator(data.GetEnumerator()));

    mockSet.As>()
        .Setup(m => m.Provider)
        .Returns(new TestDbAsyncQueryProvider(data.Provider));

    mockSet.As>().Setup(m => m.Expression).Returns(data.Expression);
        mockSet.As>().Setup(m => m.ElementType).Returns(data.ElementType);
    mockSet.As>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());

    var mockContext = new Mock();
    mockContext.Setup(c => c.Blogs).Returns(mockSet.Object);

    var service = new BlogService(mockContext.Object);
    var blog = await service.GetBlog(2);

    Assert.AreEqual("ZZZ", blog.Name);
}

但是当GetBlog从我的测试方法调用时,await _context.Blogs.FindAsync(blogId);抛出一个NullReferenceException atTestingDemo.BlogService.d__5.MoveNext()

有关如何FindAsync使用MSDN文章中的测试方法调用方法的单元测试的任何建议:http://msdn.microsoft.com/en-US/data/dn314429#async？