Calculated value is incorrect

My form computes test results. It has coumns for 4 tests. All of the calculations work properly except the seventh one in each column. That one will give a bogus result until another data entry is made any where on the form. Can someone tell me why?

The script in the problem block is

var v1 = +getField("Field15a").value;
var v2 = +getField("Cyla").value;

event.value = (v2 !== 0) ? (v1 / v2) : "";