Wordpress user creation via profile "account creation required" gives exception

When I have a profile set up, and use the "account creation required" option in the advanced settings I get an error when someone tries to use the contribution page I've set up to allow registrations.

It creates the contact in CiviCRM, but it does not create the user in the wordpress installation, and the screen that loads is:

Fatal error: Call to undefined method CRM_Utils_System_WordPress::createUser() in wp-content\plugins\civicrm\civicrm\CRM\Core\BAO\CMSUser.php on line 217

I imagine selecting "optional" will give the same results.