Why is setAttribute not changing my xml?

I have the following xml (it's a snippet of larger xml):

<fillet index="3">
  <imgsrc>somesource</imgsrc>
  <width>0.31</width>
  <cid>9351</cid>
  <sku>somesku</sku>
</fillet>

I want to change the index="3", to index="2"

This is my javascript code:

var parser = new DOMParser(); //create a new DOMParser
var doc = parser.parseFromString(mattes_get_mattes_xml(), "application/xml"); //convert the string to xml
if ("undefined" != typeof(doc.getElementsByTagName('fillet')[index]))
{
  if (mattes_remove_layer_call === false)
  {
    doc.getElementsByTagName('fillet')[index].innerHTML = "";
  }
  else
  {
    console.log(doc.getElementsByTagName('fillet')[index].getAttribute("index"));
    doc.getElementsByTagName('fillet')[index].setAttribute("index", (index - 1)); //switch the index number for the top layer
    console.log(doc.getElementsByTagName('fillet')[index].getAttribute("index"));
  }
}
var serializer = new XMLSerializer(); //create a new XMLSerializer
mattes_mattes_xml = serializer.serializeToString(doc); //convert the xml back to a string

However, the setAttribute is not changing the value. I know this because the console.log statements are both outputting 3.