You need to check turnoverTransactiions.Comput(....).HasValue. If it does, then you can do the cast.


>

>        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....