Object reference not set to an instance of an object. on report viewer

Object reference not set to an instance of an object. on report viewer

the report its showing correctly

but when try to change the value of the parameter i get this error :

Object reference not set to an instance of an object

the parameter its on the left

this its the code on default.aspx.cs

            ReportDocument myReport = new ReportDocument();

            DataSet myData = new DataSet();

            MySql.Data.MySqlClient.MySqlConnection conn;

            MySql.Data.MySqlClient.MySqlCommand cmd;

            MySql.Data.MySqlClient.MySqlDataAdapter myAdapter;

            conn = new MySql.Data.MySqlClient.MySqlConnection();

            cmd = new MySql.Data.MySqlClient.MySqlCommand();

            myAdapter = new MySql.Data.MySqlClient.MySqlDataAdapter();

            conn.ConnectionString = "server=127.0.0.1;uid=root;" +

            "pwd=;database=dinis;";

            try

            {

              cmd.CommandText = "SELECT * FROM user";

              cmd.Connection = conn;

              myAdapter.SelectCommand = cmd;

              myAdapter.Fill(myData);

              string fullReportPath = Server.MapPath("\\Reports\\CrystalReportodbcdinis.rpt");

//  @UserId is the parameter you added at the design of the crystalReport and userid is the

              myReport.Load(fullReportPath);

   // myReport.Database.Tables(0).SetDataSource(myData.Tables(0));

   // myReport.Database.Tables(1).SetDataSource(myData.Tables(1));

              ParameterDiscreteValue paramDV_UserId = new ParameterDiscreteValue(); // Step 1

              int userid = 8;

              paramDV_UserId.Value = userid; // step 2:userid is the value for the parameters

             // myReport.SetParameterValue("Utilizador",7);

              ParameterValues currentParameterValues = new ParameterValues();

              ParameterDiscreteValue parameterDiscreteValue = new ParameterDiscreteValue();

              parameterDiscreteValue.Value = userid.ToString();

              currentParameterValues.Add(parameterDiscreteValue);

              myReport.DataDefinition.ParameterFields["Utilizador"].ApplyCurrentValues(currentParameterValues);

http://localhost:13131/Default.aspx.cs

    CrystalReportViewer1.ReportSource = myReport;

}

catch (MySql.Data.MySqlClient.MySqlException ex)

{

    Response.Write(ex.Message);

    //MessageBox.Show(ex.Message, "Report could not be created",

    //    MessageBoxButtons.OK, MessageBoxIcon.Error);

}