$ispaid = NULL in ShoppingCartCheckoutCompletePage hook

I wrote a simple PayEx gateway module that works fine, but when using the hook ShoppingCartCheckoutCompletePage $ispaid = NULL, although the database has it right;

| 16 | 4 | | 2013-06-10 | 2013-06-17 | 2013-06-10 20:15:48 | 19.95 | 0.00 | 0.00 | 0.00 | 19.95 | 0.00 | 0.00 | Paid | payex | |


Part of the var_dump() of $vars within the hook:
["orderid"]=>
int(16)
["ordernumber"]=>
string(10) "5618747162"
["invoiceid"]=>
string(2) "16"
["ispaid"]=>
NULL
["amount"]=>
string(5) "19.95"
["paymentmethod"]=>
string(5) "payex"

Not sure if I'm supposed to set this in the callback or not, but I pretty much do exactly as the very nice gateway module example;

if($transactionStatus == '0' || $transactionStatus == '6') {
addInvoicePayment($invoiceid,$transid,$amount,$fee ,$gatewaymodule); # Apply Payment to Invoice: invoiceid, transactionid, amount paid, fees, modulename <----

Any advice appreciated,

Zanko