Help Please

I am new to TypeMock and have been trying to figure this out. If someone could help me I would greatly appreciate it.

So I have a Method in the MasterOrderBPO class:

public PoeordersCollection GetOrders(Int32 inID)
{
   IPredicateExpression Filter = new PredicateExpression();
   Filter.Add(PoeordersFields.MasterOrderId == inID);

   PoeordersCollection Orders = new PoeordersCollection();
   Orders.GetMulti(Filter);

   return Orders;
}

and I want to run a test on it and always get the count as 1 for Orders.Count

so here is the test:

public void ValidGetOrders()
{
   bool hasErrors = true;

   Mock orderMock = MockManager.MockAll(typeof(PoeordersCollection));
   orderMock.ExpectGetAlways("Count", 1);

   PoeordersCollection order = new PoeordersCollection();

   MasterOrderBPO orderCol = MasterOrderBPO.getInstance();

   PoeordersCollection Order = orderCol.GetOrders(1);

   if (Order.Count > 0)
   {
      hasErrors = false;
   }

   Assert.IsTrue(hasErrors == false, "The Master Order: " + moValid.MasterOrderId + " - Does Not Exist");
}

however, when it hits GetMulti I get:

failed: System.NullReferenceException : Object reference not set to an instance of an object.
   at SD.LLBLGen.Pro.ORMSupportClasses.CollectionCore`1.Clear()

What am I doing wrong? Please help me.