An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll

private void btnmodi_Click(object sender, EventArgs e)
        {
            if (radGridView1.RowCount <= 0)
            {

                MessageBox.Show(" Select Record to Updae","" );
                return;
            }

            if (blUpdate == true)
            {
                btnmodi.Text = "&Update";
                blUpdate = false;


                txtCode.Text = radGridView1.SelectedCells[0].Value.ToString();
                txtcatagory.Text = radGridView1.SelectedCells[1].Value.ToString();



                btndele.Enabled = false;
                btnsav.Enabled = false;
                txtCode.Enabled = false;
                txtCode.Focus();
            }
            else
            {
                btnmodi.Text = "&Edit";
                blUpdate = true;

                {
                    SqlConnection cn = new SqlConnection(Program.myConnection);
                    string sql = "update student set code=@code,catagory=@catagory,  where code=" + radGridView1.SelectedCells[0].Value.ToString();

                    SqlCommand cmd = new SqlCommand(sql, cn);



                    cmd.Parameters.AddWithValue("@code", txtCode.Text.Trim());
                    cmd.Parameters.AddWithValue("@catagory", txtcatagory.Text.Trim());
              


                    try
                    {
                        cn.Open();
                        cmd.ExecuteNonQuery();
                        MessageBox.Show("Data has been update successfully in database.", "Update Record", MessageBoxButtons.OK, MessageBoxIcon.Information);


                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message);
                    }
                    finally
                    {
                        cn.Close();
                        cmd.Dispose();

                        fillGrid();
                        btnsav.Enabled = true;
                        btndele.Enabled = true;
                        txtCode.Enabled = true;
                        txtCode.Focus();
                        

                    }
                }
            }
        }