Registering for an event with a waitlist yields fatal error

Online registration for an event with a waitlist, using PayPal WPS for the processor, yeilds a fatal error after confirming registration information.

If the event also has Pay later enabled, choosing PayPal as the payment method causes the registration to go through, except displaying the Pay Later instructions as though pay later had been selected.

On the demo site, the event is here:
http://drupal.demo.civicrm.org/civicrm/event/info?reset=1&id=9

The demo site yields no error information, just a white screen. The site I'm testing this on yields the following error:

Fatal error: Call to a member function doDirectPayment() on a non-object in [path/to/civicrm]/CRM/Event/Form/Registration/Confirm.php on line 588.