How to mock method with ViewModel in MVC4 with Entity Framework 4.0

I want to test the following method:

public ActionResult Index()
{
    var transactions = db.Transactions.Include(t => t.User)
                            .GroupBy(t => t.UserId)
                            .Select(group => new TransactionViewModel
                            {
                                User = group.FirstOrDefault().User.FullName,
                                UserId = group.FirstOrDefault().UserId,
                                Total = (group.Sum(t => t.TransactionAmount))
                            });

    // Show lowest balance first
    return View(transactions.ToList());
}

Here the Transaction model has a list of Orders, has a foreign key to User and some more properties, see:

public class Transaction
{
    public int TransactionId { get; set; }
    public DateTime Date { get; set; }
    public int UserId { get; set; }
    public List<Order> Orders { get; set; }
    public decimal TransactionAmount { get; set; }
    public virtual User User { get; set; }
}

The TransactionViewModel looks as follows:

public class TransactionViewModel
{
    public string User { get; set; }
    public int UserId { get; set; }
    public decimal Total { get; set; }
}

and is used to calculate the Total of different transactions belonging to a user.

To test this method I have a FakeDbSet and use a FakeContext (which both work in tests of other controllers) in the following Setup:

[TestClass]
public class TransactionControllerTest
{
    TransactionController trController;

    [TestInitialize]
    public void TransactionControllerTestInitialize()
    {
        // Arrange 
        var memoryTransactionItems = new FakeDbSet<Transaction>
        {
           new Transaction {
               Date = DateTime.Today,
               TransactionAmount = 5.10M,
               UserId = 1,
               Orders = new List<Order>{
                    // Categorie 2 and confirmed
                    new Order { OrderId = 2, 
                                UnitPrice = 2.00M, 
                                Quantity = 1, 
                                Date = DateTime.Today, 
                                IsConfirmed = true, 
                                User = new User { 
                                    Name = "Kees", 
                                    FullName="Kees Piet", 
                                    Email = "[email protected]", 
                                    isAvailable = true, 
                                    UserId = 1 
                                }, 
                                Product = new Product {
                                    Category = new Category {
                                        CategoryId = 2, 
                                        Name = "Categorie2"
                                    }, 
                                    Name = "Testproduct2",
                                    Price = 2.00M,
                                    Visible = true
                                }
                    },
                    // Categorie 2 and confirmed
                    new Order { OrderId = 2, 
                                UnitPrice = 1.00M, 
                                Quantity = 1, 
                                Date = DateTime.Today, 
                                IsConfirmed = true, 
                                User = new User { 
                                    Name = "Jan", 
                                    FullName="Jan Piet", 
                                    Email = "[email protected]", 
                                    isAvailable = true, 
                                    UserId = 2 
                                }, 
                                Product = new Product {
                                    Category = new Category {
                                        CategoryId = 2, 
                                        Name = "Categorie2"
                                    }, 
                                    Name = "Testproduct2",
                                    Price = 3.10M,
                                    Visible = true
                                }
                    }
               }
           }
        };


        // Create mock units of work
        var mockData = new Mock<FakeContext>();
        mockData.Setup(m => m.Transactions).Returns(memoryTransactionItems);

        // Setup controller
        trController = new TransactionController(mockData.Object);
    }

    [TestMethod]
    public void TestTransactionIndex()
    {
        // Invoke
        var viewResult = trController.Index() as ViewResult;
        var transactionsFromView = (IEnumerable<TransactionViewModel>)viewResult.Model;

        // Assert
        Assert.AreEqual(1, transactionsFromView.Count(),
            "The amount of transactions added to the Index View should be 1.");
    }
}

When I run the TestTransactionIndex I get the following error:

Test Name: TestTransactionIndex Test Outcome: Failed Test Duration: 0:00:30.6276475

Result Message: Test method Tests.Controllers.TransactionControllerTest.TestTransactionIndex threw exception: System.NullReferenceException: Object reference not set to an instance of an object. Result StackTrace: at lambda_method(Closure , IGrouping2 ) at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext()
at System.Collections.Generic.List1..ctor(IEnumerable1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source) at Controllers.TransactionController.Index()

I find this strange since I setup my mock units in the proper way. I hope someone can explain how I can properly send the FakeDbSet<Transaction> to the view and not get a NullReferenceException.

/Edit As requested, here are the contructors for TransactionController:

private IContext _context;

public TransactionController()
{
    _context = new Context();
}

public TransactionController(IContext context)
{
    _context = context;
}