isset($payment['pk_i_id'] returns FALSE

[ratio91]

I was delighted when the lated Update of the Payment Plugin was released on October 30th. http://market.osclass.org/plugins/payments_52
It removed a lot of problems I had with this plugin (showing only a Server 500 Error instead of returning after payment)

However, there is still one big issue left to solve:
When returning from PayPal to the website (via IPN), what shows up is an info message saying that the payment will be processed.
Unfortunately the parameter b_premium of the ITEM is not being changed to 1 from 0. Therefore, the ITEM does not become a Premium ad :/

Looking into the return.php file, the reasons seems to be that the payment id does not exist: :-
isset($payment['pk_i_id']

Can anyone here help me with this issue?
Thanks!!!