>You need to check turnoverTransactiions.Comput(....).HasValue. If it does, then you can do the cast.
>
You're right. If empty, it's not null, but DBNull.Value. That's why nullable types are of no use here.

Many thanks!
>>

>>        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....