node.js files.upload.path is undefined

I'm working through the node beginner book and I'm at the final step. At this code:

var form = new formidable.IncomingForm();
form.parse(request,function(error,fields,files){
    fs.rename(files.upload.path,"/tmp/test.png",function(error){
        if(error){
            fs.unlink("/tmp/test.png");
            fs.rename(files.upload.path,"/tmp/test.png");
        }

    });
});

terminal tells me that files.upload is undefined - I'm not sure why - can anyone tell me?