Error while creating XML file in JAVA

Am reading data from MySQL database and writing it to XML file. AM getting following error "HIERARCHY_REQUEST_ERR: An attempt was made to insert a node where it is not permitted. " Any Ideas how to solve this

private static Document buildCustomerXML(ResultSet _customerRS, ResultSet contractRS) throws Exception 
  { 

  Document xmlDoc = new DocumentImpl(); 

  /* Creating the root element */ 
  
  Element rootElement1 = xmlDoc.createElement("Contracts"); 
  xmlDoc.appendChild(rootElement1);
  while(contractRS.next()){
          Element contract = xmlDoc.createElement("Contract"); 

            /* Build the CustomerId as a Attribute*/ 
            contract.setAttribute("ID", _customerRS.getString("ID")); 
            rootElement1.appendChild(contract); 
  }
  Element rootElement = xmlDoc.createElement("Employees"); 
  xmlDoc.appendChild(rootElement);
  while(_customerRS.next()) 
   { 
           
    Element employee = xmlDoc.createElement("Employee"); 

    /* Build the CustomerId as a Attribute*/ 
    employee.setAttribute("id", _customerRS.getString("id")); 

    /* Creating elements within customer DOM*/ 
    Element contractid = xmlDoc.createElement("ContractID"); 
    Element lastName = xmlDoc.createElement("Name"); 
    Element skills = xmlDoc.createElement("Skills");
    Element skill = xmlDoc.createElement("Skill");
     /* Populating Customer DOM with Data*/ 
    contractid.appendChild(xmlDoc.createTextNode(_customerRS.getString("Contractid"))); 
    lastName.appendChild(xmlDoc.createTextNode(_customerRS.getString("last"))); 
    
    skill.appendChild(xmlDoc.createTextNode(_customerRS.getString("Skill"))); 
    
    /* Adding the firstname and lastname elements to the Customer Element*/ 
    employee.appendChild(contractid); 
    employee.appendChild(lastName);
   
    skills.appendChild(skill);
    employee.appendChild(skills);
   
    

    /* Appending Customer to the Root Class*/ 
    rootElement.appendChild(employee); 
   } 
  return xmlDoc; 
  }