Adding attribute to XML Node always fail

I always get exception below when try to add attribute, why it's not working?

The prefix '' cannot be redefined from '' to 'http://ws.plimus.com' within the same start element tag.

Code

var docXml = new XElement("param-encryption", 
    new XAttribute("xmlns", "http://ws.plimus.com"),
    new XElement("parameters"));

var s = docXml.ToString();

I want to get result like

<param-encryption xmlns="http://ws.plimus.com">
  <parameters>


  </parameters>
</param-encryption>