Shipping error?

[aap411vm]

Hi there,

After placing an order I got this error:
Warning: Parameter 1 to plgVmShipmentWeight_countries::plgVmOnSelectCheckShipment() expected to be a reference, value given in /usr/home/jules/domains/fourmedia.nl/public_html/beukelaer/libraries/joomla/event/event.php on line 71

I am using the onepage checkout from rupostel.