Loop on all items in a collection

I need to loop on all items in a collection in a VB.NET program that is using the InDesign API. For example, I want to loop on all master spreads in a document. Here is my sample code:

    Dim I As Integer = 0

    Dim MyApp As InDesign.Application = Nothing

    Dim MyDoc As InDesign.Document = Nothing

    Dim MyPage As InDesign.Page = Nothing

    MyApp = CType(Activator.CreateInstance(Type.GetTypeFromProgID("InDesign.Application"), True), InDesign.Application)

    MyDoc = CType(MyApp.Documents.Add(False), InDesign.Document)

    With MyDoc

      With .DocumentPreferences

        .PageHeight = "8.5i"

        .PageWidth = "5.5i"

      End With

      With .ViewPreferences

        .HorizontalMeasurementUnits = InDesign.idMeasurementUnits.idPoints

        .VerticalMeasurementUnits = InDesign.idMeasurementUnits.idPoints

      End With

      If .MasterSpreads.Count > 0 Then

        For I = 0 To .MasterSpreads.Count - 1

          Dim ThisMasterSpread As InDesign.MasterSpread = CType(.MasterSpreads.Item(I), InDesign.MasterSpread)

        Next I

      End If

    End With

When it executes, I get an error on the last Dim statement that defines ThisMasterSpread. It highlights .MasterSpreads.Item(I) and gives the following error:

COMException was unhandled: The requested member of the collection does not exist.

How do I successfully loop on all master spreads?

Thanks!