Error !! with Crystal report in VS 2005 (C#) (Help me Please who has a ploblem same me)

Question

This is  my error !!

Logon failed. Details: crdb_adoplus : Object reference not set to an instance of an object. Error in File C:\DOCUME~1\EMWORK~1\ASPNET\LOCALS~1\Temp\CrystalReport {E313040D-019A-49CE-A3E4-EACD36CB486B}.rpt: Unable to connect: incorrect log on parameters.

I create crystal report using DataSet  and use tools in VS 2005 such as CrystalReportViewer and CrystalReportSorce. but it not to load Data to show me. I hope maybe someone can help me. Thank you .

This is my sample code in Report.aspx 

using System;

using System.Data;

using System.Configuration;

using System.Collections;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Web.UI.HtmlControls;

using System.Data.SqlClient;

using CrystalDecisions.CrystalReports.Engine;

public partial class Report : System.Web.UI.Page

{

                 protected void Page_Load(object sender, EventArgs e)

                 {

                        string dsn = ConfigurationManager.ConnectionStrings ["NorthwindConnectionString"].ConnectionString;

                        SqlConnection cn = new SqlConnection(dsn);

                        string selectCmd = @"SELECT CategoryID, CategoryName, Description, Picture FROM Categories";

                        SqlDataAdapter da = new SqlDataAdapter(selectCmd, cn);

                        DataSet ds = new DataSet("Rpt");

                        da.Fill(ds, "Table1");

                    

                        ReportDocument myReportDocument = new ReportDocument();

                        myReportDocument.Load(Server.MapPath(@"CrystalReport.rpt"));

                        myReportDocument.SetDataSource(ds);

                        CrystalReportViewer1.ReportSource = myReportDocument;

                       //CrystalReportViewer1.DataBind();

                        CrystalReportViewer1.DisplayToolbar = false;

 

                }

}

=================================================Thks.