Trying to set up Appointments+ to schedule parent assists

Trying to set up Appointments+ to schedule parent assists for a preschool. These will not have an associated cost. I set the price to $0 but am getting the error

Warning: Missing argument 3 for App_Membership2::get_price() in /home/benfineman/preschool.benfineman.com/wp-content/plugins/appointments/includes/class_app_membership2.php on line 89

http://preschool.benfineman.com/make-an-appointment/

Can you help?

Thanks,
Ben