[Plugin: WP-Chargify] Wp-Chargify Catchable fatal error

This plugin is mostly working, however, upon signing up, my customers are getting this:

Catchable fatal error: Object of class WP_Error could not be converted to string in /xxx/wp-includes/formatting.php on line 2773

The charges go through, but this is a pretty annoying error. Anyone know what’s going on?