Appointments+ debug error

Hi,

I've noticed there is an error logged in debug.log (when debugging is on) after a new appointment has been created by a logged in user, with a valid email address filled out in the appointment form:

[17-Oct-2017 17:42:15 UTC] WP_Error Object
(
    [errors] => Array
        (
            [wp_mail_failed] => Array
                (
                    [0] => You must provide at least one recipient email address.
                )

        )

    [error_data] => Array
        (
            [wp_mail_failed] => Array
                (
                    [to] => Array
                        (
                            [0] =>
                        )

                    [subject] =>
                    [message] =>
                    [headers] => Array
                        (
                            [MIME-Version] => 1.0
                        )

                    [attachments] => Array
                        (
                        )

                    [phpmailer_exception_code] => 2
                )

        )

)

Seems like this is about an email that should be sent to the visitor/user, not the service provider...

Is this a bug or something missing in my settings somewhere?