Country field not syncing from WP to Mailchimp

I've setup a registration form on my site with Firstname, Lastname, email and Country. I've setup the same fields in mailchimp. All are syncing correctly except for the Country field. I've ensured that the Mailchimp country field is setup with the merge tag 'Country', but it won't sync this field. Let me know if there is an additional option that needs to be configured in order for these to sync.