Invoice Pro 1.52for OC 2.2.0.0

Hi

Hope somebody can help me with this:

I got this error message in the shop when I activate the Invoice Pro Fee extension.

"Warning: Parameter 1 to ModelTotalInvoiceProFee::getTotal() expected to be a reference, value given in /var/www/html/system/storage/modification/system/engine/loader.php on line 150?"

I have double and tripple checked all forums and so on but culdnt find any solution.

Thanks in advance