Sum function script not updating field when variables being blank

Earlier, I asked how to hide values if the the value=0 and got the correct script to put in.

The script I used for this was:

var amount = Number(this.getField("Qty2").value) * Number(this.getField("UP2").value);

event.value = (amount==0) ? "" : amount;

However, the sum function for the "Subtotal" field is now not updating. I am using the sum function in the calculator tab. Am I suppose to put in a script now or can I still use the sum function? Please see the snapshot below. Thanks!