Unknown column 'Array'

Hello,

I wonder why I get this error message? I thought I already define the array.

Error Number: 1054
Unknown column 'Array' in 'where clause'
SELECT `id` FROM `users` WHERE `referral_code` = Array
Filename: C:/xampp/htdocs/iPengen_Marketing_new/api/system/database/DB_driver.php
Line Number: 691



users_model.php

     function FindUserIdChild($data = array())
        {
        // referral code bkn marketing leader
        $this->db->select('id');
        $this->db->where('referral_code', $data);
        $this->db->from($this->table);
        return $this->db->get()->result();

        }

controllers/marketing.php
       
        $ref_code = 'ABC15';

        $dna['dna'] = $this->users_model->FindDnaCodeChild($ref_code);
        
        $users2 = $this->users_model->FindUserIdChild($dna);

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