Base Collection

I have Units Called ATPUnit with following properties :

    ReadOnly Property Unit() As Integer
    ReadOnly Property Prog() As Integer
    ReadOnly Property RLU() As Integer
    ReadOnly Property Heure() As Date
    ReadOnly Property Test() As Integer


I want to create an ATPUnitColl. So here we go

Public Interface intATPCollUnit

    Default ReadOnly Property Item(ByVal index As Integer) As MeasureUnit
    ReadOnly Property ItemAtTest(ByVal index As Integer) As Integer
    Sub Add(ByVal theATPUnit As ATPUnit)
    Sub Remove(ByVal MeasUnit As MeasureUnit)

End Interface


I have problem on runtime when i try to add an ATPUnit to my collection :

    Public Sub Add(ByVal theATPUnit As ATPUnit) Implements intATPCollUnit.Add
        List.Add(theATPUnit)
    End Sub


Any idea ? Anything missing ? Do I need to "link" the collection via a specific attribute from my collector ?