When i updating grid view error occurs

protected void ResultGridView_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
  TextBox txtFName = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtFName");
        TextBox txtLName = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtLName");
        TextBox txtCity = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtCity");
        TextBox txtState = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtState");
        TextBox txtCountry = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtCountry");
        TextBox txtDescription = (TextBox)ResultGridView.Rows[e.RowIndex].FindControl("txtDescription");
 
 cmd.Connection = con;
 cmd.CommandText = "Update Vendors Set VendorFName = '" + txtFName.Text + "',VendorLName = '" + txtLName.Text + "',VendorCity = '" + txtCity.Text + "',VendorState = '" + txtState.Text + "',VendorCountry = '" + txtCountry.Text + "',VendorDescription = '" + txtDescription.Text + "' where VendorId = " + ResultGridView.DataKeys[e.RowIndex].Values[0].ToString() + "";
        con.Open();
        cmd.ExecuteNonQuery();
        ResultGridView.EditIndex = -1;
        FillVendorGrid();
        con.Close();