Using the new Initialize operation

I am trying to use the new Intialize operation of the 2.5 Beta interface to create an invoice based on a sales order.  Here is the VB.NET code I have written to do this:

        Dim iir As New InitializeRecord
        Dim so As New InitializeRef
        so.internalId = 7459
        so.type = InitializeRefType.salesOrder
        so.typeSpecified = True
        iir.type = InitializeType.invoice
        iir.reference = so
        Dim result As ReadResponse = service.initialize(iir)

When I run this code, the initialize operation returns this error message:
You have an invalid sales order 7459 or the order is already closed.

7459 is the correct internalId for the sales order, and the sales order is definitely not closed.  I get the same error if I try to create an ItemFulfillment the same way.  Any ideas?