Simple Yes/No checkbox

I’m trying to create a Boolean field on a user profile form. I want the user to be able to be able to tick a checkbox (or select yes/no from radio buttons) to show they’ve read and agree to a statement.

This needs to be able to sync with MailChimp, for which I’ve already set up the syncing. The problem I’m having is that UM doesn’t seem to want to use a simple Boolean, preferring the fascinating option of storing the value as an array.

I’ve tried everything but my brain is now starting to hurt.

Any clues where I’m going wrong?