Hidden Groups in MailChimp Don't Display in Hustle

I’m trying to use the Groups function of MailChimp to assign when someone subscribes through Hustle, but I don’t want the groups visible to the user. If I make the Group hidden in Mailchimp, it doesn’t display the options to select in the Hustle Plugin. If I make the Group not hidden in Mailchimp, it displays fine in the plugin, but then also shows to the user.

FYI, given the Hustle plugin has this text in it:

What you select here will appear pre-selected for users. If this is a hidden group, the interest will be set but not shown to users.

I’m guessing Hidden groups should indeed work there…

Would there be a way to simply check an option in Hustle to treat a non-hidden group as a hidden group, ie: letting you select it in the plugin, and not display it to the user?

Also, even though the field names are correct, I’m getting the Invalid Email Address error when a user submits the opt-in.