XML : Xml request header with root element in swift

I've been trying to make xml request using Alamofire. However, I got <soap:Text xml:lang="en">Server was unable to process request. ---> Root element is missing.</soap:Text> error. Actually, this is my first experience about xml request in ios. How can I achieve to make this request without issue. Here is my code;

  Alamofire.request(.POST, "http://example.com/WS.asmx?op=AuthenticateUser", parameters: nil, encoding: .PropertyList(.XMLFormat_v1_0, 0), headers: ["Username": "username", "Password": "password" , "AuthenticatedToken" : "123123"])                      .responsePropertyList { response in       let xml = SWXMLHash.parse(response.data!)                          print(xml)   }    

What am I doing wrong ? Could you help me please ?

Thank you for your answers

Best regards