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>
Jon Skeet
people
quotationmark

This simplest approach is to let LINQ to XML do this automatically by specifying the namespace in the element name:

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

Result of docXml.ToString():

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

people

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