Pro Sites PHP Error

I'm getting the following error when a user tries to upgrade via Pro Sites:

PHP Fatal error: Call to undefined method ProSites_Gateway_PayPalExpressPro::checkout_url() in /var/www/html/wp-content/plugins/pro-sites/pro-sites-files/gateways/gateway-paypal-express-pro.php on line 1334

I'm looking into it right now, but let me know if you're aware of this issue, or if anyone else is having it.