I am using sum function to sum shipping cost from multiple orders. If I have 2 Orders and both have shipping cost, sum function is totaling properly, but if if one of the Orders does not have shipping, then the shippingcost element will not be there. In this case one order did not have shipping cost so it did not have the shippingcost element. the other order had shippingcost. In this case the sum function is not summing the values. How can this be resolved?