用多选框显示数据的时候怎么会出现这样的问题,求救!

运行的时候当我选一个的时候可以显示,而在运行第二个的时候出现错误,不知道为什么,看代码也没有什么地方错误,或许是有一些细节没有注意到,查了好久也没有查到.谢谢各位帮助啊 代码是这样的.
<%@ Page language="c#" Codebehind="WebForm1.aspx.cs" AutoEventWireup="true" Inherits="thirdfile.WebForm1" %>
<%@ import namespace="System.Data"%>
<%@ import namespace="System.Data.SqlClient"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
<HEAD>
<title>WebForm3</title>
<meta content="Microsoft Visual Studio .NET 7.1" name="GENERATOR">
<meta content="C#" name="CODE_LANGUAGE">
<meta content="JavaScript" name="vs_defaultClientScript">
<meta content="http://schemas.microsoft.com/intellisense/ie5" name="vs_targetSchema">
</HEAD>
<body MS_POSITIONING="GridLayout">
<form id="Form1" method="post" runat="server">
<asp:label id="Label1" style="Z-INDEX: 101; LEFT: 56px; POSITION: absolute; TOP: 72px" runat="server">Using Radio Buttons</asp:label>
<asp:checkboxlist id="cbx" style="Z-INDEX: 102; LEFT: 56px; POSITION: absolute; TOP: 128px" runat="server"
RepeatColumns="3" AutoPostBack="True" OnSelectedIndexChanged="subListChange"></asp:checkboxlist>
<asp:DataGrid id="dg" style="Z-INDEX: 103; LEFT: 64px; POSITION: absolute; TOP: 216px" runat="server"></asp:DataGrid></form>
<script language="C#" runat="server">
private void Page_Load(object sender,System.EventArgs e)
{
   if(!IsPostBack)
   {
string str_conn="Data Source=YUCHENGGANG;Initial Catalog=Northwind;Integrated Security=SSPI";
SqlConnection conn=new SqlConnection(str_conn);

string sql_str="Select LastName,EmployeeID from Employees order by LastName;";
SqlCommand comm=new SqlCommand(sql_str,conn);

conn.Open();
cbx.DataSource=comm.ExecuteReader();
cbx.DataValueField="EmployeeID";
cbx.DataTextField="LastName";
cbx.DataBind();
conn.Close();

   }
}private void subListChange(object sender,System.EventArgs e)
{
Response.Write("subListChange triggered:");

string strWhereClause="";

foreach(ListItem liThisOne in cbx.Items)
{
if(liThisOne.Selected)
{
strWhereClause+="EmployeeID="+liThisOne.Value+"or";

}
}

Response.Write("strWhereClause=<br/>"+strWhereClause+"<hr/>");

if(strWhereClause.Length>0)
{
dg.Visible=true;

string str=strWhereClause.Substring(0,strWhereClause.Length-4);
strWhereClause="where"+str;

string str_conn="Data Source=YUCHENGGANG;Initial Catalog=Northwind;Integrated Security=SSPI";
SqlConnection conn=new SqlConnection(str_conn);
string sql_str="Select TitleOfCourtesy,FirstName,LastName,Country,Region,City,Notes  from Employees "+strWhereClause;

SqlCommand comm=new SqlCommand(sql_str,conn);

Response.Write("strSQLForGrid=<br/>"+sql_str+"<hr/>");

conn.Open();
dg.DataSource=comm.ExecuteReader();
dg.DataBind();
conn.Close();


}
else
 dg.Visible=false;
}
</script>
</body>
</HTML>