Windows Form DataGridView CheckBox checked Error: Object cannot be cast from DBNull to other types

foreach (DataGridViewRow row in dgvProductdiscount.Rows)
{
    if (Convert.IsDBNull(row.Cells["chkbox"].Value))
    {
    }
    else
    {
        isSelected = Convert.ToBoolean(row.Cells["chkbox"].Value);
        if (isSelected)
            if ((Convert.ToBoolean(row.Cells["chkbox"].Value) == true))
            {
                infoproduct.ProductId = decimal.Parse(row.Cells["productId1"].Value.ToString());
                vouchertype = cmbVoucherType.Text;
                infoproduct.VoucherTypeId = vouchertype;
                DataTable dtdisc = new DataTable();
                dtdisc = spproductdiscount.ProductDiscountselect(infoproduct);
                if (dtdisc.Rows.Count > 0)
                {
                    spproductdiscount.ProductDiscountdelete(infoproduct);
                }
            }
    }
}