Declaring private classes in the thisaddin.vb module

[Visisthebest]

When declaring simple classes that I use as pure data objects in a list (to execute a DropMany later), for instance:

Private Class ShapeDropToDoItem
    Public MasterToDrop As Visio.Master
    Public X As Double
    Public Y As Double
End Class

I find that I can declare this class as Public but not as Private (even though I do not want to use this class outside the thisaddin.vb module.

When I use Private rather than Public, I get this error:
Error   BC31089   Types declared 'Private' must be inside another type.

Why do I get this error and how can I solve it?

[Visisthebest]

I guess I need to declare it as a Friend class.