在VB.NET中使用LINQ to XML更新XML元素

这是代码：

Dim xmldoc As XDocument = XDocument.Load(theXMLSource1)
        Dim ql As XElement = (From ls In xmldoc.Elements("LabService") _
                Where CType(ls.Element("ServiceType"),String).Equals("Scan") _
                Select ls.Element("Price")).FirstOrDefault


        ql.SetValue("23")
        xmldoc.Save(theXMLSource1)

这是XML文件：

<?xml version="1.0" encoding="utf-8"?>
<!--Test XML with LINQ to XML-->

<LabSerivceInfo>

  <LabService>
    <ServiceType>Copy</ServiceType>
    <Price>1</Price>
  </LabService>

  <LabService>
    <ServiceType>PrintBlackAndWhite</ServiceType>
    <Price>2</Price>
  </LabService>

</LabSerivceInfo>

但是,我收到此错误消息：

Object reference not set to an instance of an object.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Error line:ql.SetValue("23")

你能告诉我这是什么问题吗？谢谢.