public static void addTurnover(D010Invoices.Invoice invoice)
        {        
            var turnoverTransactions = D010.loadTurnover(invoice.document);
            decimal total = 0;
            decimal? totalPaid = 0;
            if (turnoverTransactions.Rows.Count > 0)
            {
                total = (decimal)turnoverTransactions.Compute("sum(amount)", "true");
                totalPaid = (decimal?)turnoverTransactions.Compute("sum(amount)", "document like 'AP*'");
                if (totalPaid==null)
                {
                    totalPaid = 0;
                }

            }

.... so why do I get a casting error in the totalPaid line when the document does not start with "AP"?

*** UPDATE ***
Because DataTable.Compute() returns DBNull.Value and not null....