asp.net 异常：Message="Value cannot be null.\r\nParameter name: g"

ForumTableAdapters.tbTopicTableAdapter topic = new ForumTableAdapters.tbTopicTableAdapter();
            Response.Write(Request.QueryString["TopicID"].ToString());  后来加的测试行
            rpt_Topic.DataSource = topic.GetTopic(new Guid(Request.QueryString["TopicID"]));  错误提示所在行
            rpt_Topic.DataBind();

错误具体信息

System.ArgumentNullException was unhandled by user code
  Message="Value cannot be null.\r\nParameter name: g"
  Source="mscorlib"
  ParamName="g"
  StackTrace:
       at System.Guid..ctor(String g)
       at ShowTopic.Page_Load(Object sender, EventArgs e) in e:\SOLUTION\Forum\ShowTopic.aspx.cs:line 19
       at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
       at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)
       at System.Web.UI.Control.OnLoad(EventArgs e)
       at System.Web.UI.Control.LoadRecursive()
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

上网查了好久，开始以为是iD什么的没写好，然后加断点，就在找g ,因为提示是g为null。最后看看trace，at System.Guid..ctor(String g)  ，终于找到g了，所以就是说，new Guid(Request.QueryString["TopicID"]))里面有问题。加了后来加的测试行之后，提示错误在新加的行，确定问题出在Request.QueryString["TopicID"】上。仔细检查代码，终于发现"&TopicID="写成了"&TopicID"少了=，所以提示为null，问题解决。