Custom invoice numer error

Hello,

i customized the customized invice number code at:
http://www.amember.com/docs/How_to_set_up_custom_invoice_numbering
and changed this line:

$invoice->public_id = ++$last_invoice_num ."/". date('Y');

..so that the invoice number now includes a "/" and i also tried with "-" (i need those in invoice number)
This produces an error with Offline payment(i have not tried others)...basically
the "payment instructions" page that should open is not found (i think that becouse
of those special characters in URL).

Any ideas how to solve this?

Full code:
Am_Di::getInstance()->hook->add(Am_Event::INVOICE_BEFORE_INSERT, 'invoicePublicId');
function invoicePublicId(Am_Event $event){
$last_invoice_num = Am_Di::getInstance()->store->get('last_invoice_num');
$invoice = $event->getInvoice();
$invoice->public_id = ++$last_invoice_num ."-". date('Y');
Am_Di::getInstance()->store->set('last_invoice_num', $last_invoice_num);
}