Need Help With Split And Math Functions

Code:
function finalSize(){
var size = document.getElementById('size').value;
var margin = document.getElementById('margins').value;
var mirrors = document.getElemntById('mirrorsRadio').value;

var size_array = size.split('x');
var h = size_array[0];
var w = size_array[1];

var finalH = ''
var finalW = ''

if(margin > 1){
finalH = h * margin;
finalW = w * margin;
document.getElementById('finalSize').value = finalH+'x'+finalW;
}else{
newH = h * margin;
newF = w * margin;
finalH = newH + h;
finalW = newW + w;
document.getElementById('finalSize').value = finalH+'x'+finalW;
}

}
I see it's ok, but it gives me this error:

Object doesn't support this property or method.

I don't know what I'm doing wrong. All I know is PHP, but from what little I learned on Javascript, my brain is telling me I'm right, but my experience is telling me I'm wrong. XD