how to fix this 'DropDownList1' has a SelectedValue which is invalid because it does not exist in the list of items. Parameter name: value
<pre><code>public partial class allfiles : System.Web.UI.Page
{

received re = new received();
catgories cat = new catgories();
protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostBack)
    { 
      DropDownList1.DataSource = cat.get_all_cat();
      DropDownList1.DataTextField = "اسم المستند";
      DropDownList1.DataValueField = "رقم المستند";
      DropDownList1.DataBind();
    }
}
protected void GridView1_SelectedIndexChanged1(object sender, EventArgs e)
{
    // DropDownList1.Items.Clear();
   // DropDownList1.SelectedIndex = 0;
    TextBox10.Text = GridView1.SelectedRow.Cells[2].Text;
    TextBox1.Text = GridView1.SelectedRow.Cells[8].Text;
    TextBox2.Text = GridView1.SelectedRow.Cells[9].Text;
    TextBox3.Text = GridView1.SelectedRow.Cells[10].Text;
    TextBox11.Text = GridView1.SelectedRow.Cells[11].Text;
    TextBox4.Text = GridView1.SelectedRow.Cells[12].Text;
    TextBox5.Text = GridView1.SelectedRow.Cells[6].Text;
    TextBox6.Text = GridView1.SelectedRow.Cells[7].Text;


    DropDownList1.SelectedValue = GridView1.SelectedRow.Cells[3].Text;



    TextBox8.Text = GridView1.SelectedRow.Cells[4].Text;
    TextBox9.Text = GridView1.SelectedRow.Cells[5].Text;

}
</code></pre>

<p>When a row is selected then this error is displayed:</p>

<blockquote>
  <p>'DropDownList1' has a SelectedValue which is invalid because it does
  not exist in the list of items. Parameter name: value</p>
</blockquote>