Email optional causing error

We are currently using Bookings to allow customers to book appointments with librarians. As many of our customers do not have email addresses, we have made that field optional. However, we are learning that despite being optional, the system will not allow an appointment to be created without it.

Has anyone else encountered this? We can add email to the custom fields and remove it entirely from the standard fields, but wondering if there is something I’m missing.

Thanks!