Bug in File Upload Class?

I'm trying to upload a file and overwrite if exists on server but I always get error msg like:
"File exists, overwrite is FALSE, could not save over file ./uploads/avatars/1.jpg"

Here is my code where 'overwrite' is TRUE... Any idea how to solve this?

Code:

if( isset($_FILES['userfile']['name']) ) {
    if( $_FILES['userfile']['name'] != "") {
        $config['upload_path'] = 'uploads/avatars/temp/';
        $config['allowed_types'] = 'gif|jpg|png|jpeg';
        $config['remove_spaces'] = true;
        $config['overwrite'] = true;
        $config['encrypt_name'] = true;
        $config['max_size'] = '3000';
        $config['max_width']  = '3000';
        $config['max_height']  = '3000';
        $this->load->library('upload', $config);

        if (!$this->upload->do_upload('userfile')) {
            // Failed
            $error = array('error' => $this->upload->display_errors());
            print_r($error);
            exit();
        } else {
            // Success
            $image = $this->upload->data();
            var_dump( $image );
        }
    }
}