Hustle error – Something went wrong

Subscribers are not being added to the MailChimp list. After add the email in the form, hustle gives a success message, but the error log shows:

“Something went wrong, please compare the fields…”

The opt-in and the Mailchimp list only have the email field! We are not able to add the subscribers to the list unless the opt-in have, at least, the “first name”.