Bug in Voiding Customer Payment

case ST_CUSTPAYMENT : // it's a customer payment
            if (!check_void_bank_trans($type, $type_no))
                return _('This transaction cannot be voided because the operation would decrease account balance below allowed limit in some point of account history.');
        case ST_SALESINVOICE : // it's a customer invoice
             if (is_cust_invoice_credited($type_no))
                return _('This invoice cannot be voided because it was already credited.');

    case ST_CUSTCREDIT : // it's a customer credit note
        case ST_CUSTDELIVERY : // it's a customer dispatch
            if (!exists_customer_trans($type, $type_no))
                return _('Selected transaction does not exists.');
            if ($type == ST_CUSTDELIVERY)    // added 04 Oct 2008 by Joe Hunt. If delivery note has a not voided invoice, then NO.
            {
                $vers = get_customer_trans_version($type, $type_no);
                if ($vers[$type_no] == 1) {
                    $childs = get_sales_child_lines($type, $type_no, false); // 2011-03-17 This had been changed. Joe
                    if ($childs && db_num_rows($childs))
                        return _('This delivery cannot be voided because it was already invoiced.');
                }
            }
            post_void_customer_trans($type, $type_no);
            break;

case ST_CUSTPAYMENT : // it's a customer payment
            if (!check_void_bank_trans($type, $type_no))
                return _('This transaction cannot be voided because the operation would decrease account balance below allowed limit in some point of account history.');
            post_void_customer_trans($type, $type_no);
            break;
        case ST_SALESINVOICE : // it's a customer invoice
             if (is_cust_invoice_credited($type_no))
                return _('This invoice cannot be voided because it was already credited.');
            post_void_customer_trans($type, $type_no);
            break;