DcatAdmin 多對多關聯是，multipleSelect 報錯 Array to string conversion

class Activity extends Model
{
    // 定義關聯關係
    public function adminUsers()
    {
        // 中間表
        $pivotTable = 'activity_admin_users';
        // 關聯模型類名
        $relatedModel = AdminUser::class;
        return $this->belongsToMany($relatedModel, $pivotTable, 'activity_id', 'admin_user_id')->withTimestamps();
    }
}

$form->multipleSelect('adminUsers')
    ->options(AdminUser::pluck('username', 'id'))
    ->customFormat(function ($v) {
        // 二維陣列轉一維 ，主要就是這段
        return array_column($v, 'id');
    });