Mocking an EF DBSet

Hello, I am trying to write a simple unit test to get me started with TypeMock and am 
having trouble figuring out how to mock an EF DBSet. I have looked at the "Mocking DBSet"
question and did not actually see a clear answer so I am posting my own question. 

I am trying to test the "GetReportDefinitionComboBoxItemsByReportType" method below, and 
in doing so I will need to mock the dbContext.ReportDefinitionEntities. I have pasted 
what I have tried below the function that I am trying to test so you can see what I am 
doing. When I run this test, I get a System.NullReferenceExcpetion, I am guessing on the
dbContext.ReportDefinitionEntities. 

What am I doing wrong?

Public Function GetReportDefinitionComboBoxItemsByReportType(ByVal reportType As ReportTypes) As List(Of StandardComboBoxItem)

 Using dbContext As New MyDataContext(_connectionString)

  ' ReSharper disable once AccessToDisposedClosure
  Dim query = From reportDefinitionEntity In dbContext.ReportDefinitionEntities
     Where reportDefinitionEntity.ReportType = reportType
     Order By reportDefinitionEntity.Name
     Select New StandardComboBoxItem With
     {
     .DisplayText = reportDefinitionEntity.Name,
     .Value = reportDefinitionEntity.ReportDefinitionId
     }

  Return query.ToList()

 End Using

End Function

 <TestMethod()>
Public Sub GetReportDefinitionComboBoxItemsByReportType()

 _fakeDbContext = Isolate.Fake.AllInstances(Of MyDataContext)()
 _reportsDal = New ReportsDal()


 Dim entities As DbSet(Of ReportDefinitionEntity) = _fakeDbContext.Set(Of ReportDefinitionEntity)()

 entities.Add(New ReportDefinitionEntity() With {.Name = "Name C", .ReportDefinitionId = 3, .ReportType = ReportTypes.BusinessRegistrationForm})
 entities.Add(New ReportDefinitionEntity() With {.Name = "Name A", .ReportDefinitionId = 2, .ReportType = ReportTypes.BusinessRegistrationForm})
 entities.Add(New ReportDefinitionEntity() With {.Name = "Name B", .ReportDefinitionId = 1, .ReportType = ReportTypes.BusinessRegistrationForm})

 Isolate.WhenCalled(Function() _fakeDbContext.ReportDefinitionEntities).WillReturnCollectionValuesOf(entities)
 
 Dim results = _reportsDal.GetReportDefinitionComboBoxItemsByReportType(ReportTypes.BusinessRegistrationForm)

 Assert.IsNotNull(results)
 Assert.AreEqual(results.Count, 3)

End Sub

Thank you!