C# - System.Data.Linq.ChangeConflictException - Row Not Found Or Changed

I wasted the better part of the day on this and am no closer to a understanding the issue than what I was this morning.

I am looping through a set of objects and marking them for deletion. The 2nd one always causes the above exception. tb_invoice has a FK to tb_shipment.

As always, I am probably missing something very obvious, but I have stripped out so much from this code already that there is nothing left, and I am still getting this exception. This is a local SQL 2008 instance and there is of course nothing and nobody changing the invoice in between reading them and calling SubmitChanges().

myDataContext db = new myDataContext();
IQueryable<invoiceDetail> pendingInvoices
db.GetInvoiceDetailPending();
foreach (invoiceDetail id in pendingInvoices) {
tb_shipment s = db.GetShipmentById((Guid)id.shipment_id);
db.tb_invoices.DeleteOnSubmit(
db.GetInvoiceById(s.tb_invoices.FirstOrDefault().id)); }
SubmitChanges(); // fails for the 2nd invoice
}