How to download a xml content in a xml file

Possible Duplicate:
How to download xml file in asp.net using C#

can anyone please help how to download the xml (which is in string).I am using MVC3

Mycode;

 public FileResult Download(string id)
    {  
        var model = service.GetAllDefinitions().First(x => x.ID == id);
        var definitionDetails = new StatisticDefinitionModel(model);
        string xmlString = definitionDetails.ToXml;
     //string presented xml  

        string fileName = definitionDetails.Name + ".xml";

            var stream = new MemoryStream();

            var writer = XmlWriter.Create(stream);
            writer.WriteRaw(xmlString);
            stream.Position = 0;
            var fileStreamResult = File(stream, "application/xml", fileName);
            return fileStreamResult; 

    }

but this not working.gives an error

Error:

XML document must have a top level element. Error processing resource

Thanks,