Ah. I was thinking that Weight itself was nullable. Try this:

var q  = context.EMTransactions.Where(r => r.TransactionHeaders_Id == transactionHeaders_Id).Sum(r => (decimal?) r.Weight);
// or, if you don't want q to be nullable:
var q  = context.EMTransactions.Where(r => r.TransactionHeaders_Id == transactionHeaders_Id).Sum(r => (decimal?) r.Weight) ?? 0;

>Hi Viv.
>
>>If you want null results to be treated as 0 then you could just use : Sum(r=>r.Weight ?? 0)
>
>Doesn't compile either.
>
>This on the other hand does work. Interestingly q does not return null if there no transactions in EM. So why won't it sum()?
>
>

>        public static decimal ComputeWeightDifference(int transactionHeaders_Id,PICSEntities context)
>        {
>            decimal emWeight = 0;
>            var q = context.EMTransactions.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id));
>            if ((q.Count() == 0) == false) { emWeight = q.Sum(r => r.Weight); }
>            decimal detailsWeight = context.TransactionDetails.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id)).Sum(r => r.Weight);
>            return emWeight   - detailsWeight;
>        }
>