decimal emWeight = context.EMTransactions.Where(r => r.TransactionHeaders_Id == transactionHeaders_Id).Sum(r => (decimal?)r.Weight) ?? 0;
            // 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;

This indeed works. If I'm right

Sum(r => (decimal?)r.Weight)

the (decimal?) type definition applies to Sum() because Weight is defined a non nullable in my SQL table.

Many thanks for your help.