DropDownlist Error

Question

Hi ,
    I have DropDownList and few text boxes . Showing error... Invalid Argument

  protected void btnSelect_Click(object sender, EventArgs e)

        {

            LabTestClass LB = new LabTestClass();

            DataTable dtn = LB.ShowRec(DP1.SelectedItem.Value);

            tbId.Text = dtn.Rows[0]["ID"].ToString();

            tbName.Text = dtn.Rows[0]["Name"].ToString();

            tbDesig.Text=dtn.Rows[0]["Desgn"].ToString();

            

        }

at dropdownlist


I have sqldb with ID Autogenrated which I m bind to DRopdown..
based on selected value..it show records from db.....



Plz help....