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The mailchimp_get_list and mailchimp_get_lists() functions do not return $list as an object that can be used to programmatically add subscribers. Developers need to manually/programmatically cast the return value from this function to an (object) in order to submit to mailchimp_subscribe_user.
Furthermore, developer needs to add mc_list_id field and label field, as well, in order for mailchimp_subscribe_user to return success.
I think that mailchimp_get_list should return an object that should be compatible with the mailchimp_subscribe_user function.