Adding Values To Form Text Fields

I want to do is add the values of seven text fields on a form and pop that value to an eighth field. I thought the code below would work, but I get "NaN" on the sum field.

Code:
function addHazWaste() {
document.getElementById('HazardousWaste').value = 0;
var HazWasteTotal = 0;
var HazWaste1 = 0;
var HazWaste2 = 0;
var HazWaste3 = 0;
var HazWaste4 = 0;
var HazWaste5 = 0;
var HazWaste6 = 0;
var HazWaste7 = 0;
HazWaste1 = parseInt(document.getElementById('HazardousWasteSentToLandfill').value,10);
HazWaste2 = parseInt(document.getElementById('HazardousWasteSentToWasteToEnergy').value,10);
HazWaste3 = parseInt(document.getElementById('HazardousWasteIncinerated').value,10);
HazWaste4 = parseInt(document.getElementById('HazardousWasteRecycled').value,10);
HazWaste5 = parseInt(document.getElementById('HazardousWasteReused').value,10);
HazWaste6 = parseInt(document.getElementById('BiohazardousWasteSentToLandfill').value,10);
HazWaste7 = parseInt(document.getElementById('BiohazardousWasteIncinerated').value,10);
HazWasteTotal = HazWaste1 + HazWaste2 + HazWaste3 + HazWaste4 + HazWaste5 + HazWaste6 + HazWaste7;
document.getElementById('HazardousWaste').value = HazWasteTotal;
}

If I run a separate function that just sets all form field values to zero (including the first line of this function). The summing works!!! Why won't it work the way I wrote it??? It won't work if I set the default value for the sum field to zero, either...