C#: Null reference exception thrown while using XmlDocment

A NullReferenceException is thrown by the runtime when I convert XElement into XmlNode using the following function:

public static XmlNode GetXmlNode(this XElement element)
{
    using (XmlReader xmlReader = element.CreateReader())
    {
        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.Load(xmlReader);
        xmlDoc.ChildNodes[4].InnerXml = "0.15"; ====> null reference exception occurs here
        return xmlDoc;
    }
}

How can I convert XElement to XmlNode without this problem?