add child nodes using libxmljs (Node.js) to a XML file

I'm trying to add a new child node to an xml file; so far I have this code:

var libxml = require('libxmljs');
var xml =  '<?xml version="1.0" encoding="UTF-8"?>' +
           '<root>' +
               '<child foo="bar">' +
                   '<grandchild baz="fizbuzz"><blah>grandchild content</blah></grandchild>' +
               '</child>' +
               '<child foo="bar1">' +
                   '<grandchild baz="fizbuzz">grandchild content 1</grandchild>' +
               '</child>' +
               '<child foo="bar3">' +
                   '<grandchild baz="fizbuzz3">grandchild content 3</grandchild>' +
               '</child>' +
               '<sibling>with content!</sibling>' +
           '</root>';


var xmlDoc = libxml.parseXml(xml);
var allxml = xmlDoc.root();  //store all nodes as allxml
var allNodes = xmlDoc.childNodes(); //all child nodes to array
var elem = xmlDoc.node('name1');
var newChild = libxml.Element(xmlDoc, 'new-child');
elem.addChild(newChild);

But I get the following error when I run this:

return this._root(elem);
            ^
Error: Holder document already has a root node

Does anyone know whats going on here?