code: protected function invalid(): Collection
      {
          if (!is_array($this->value)) {
              return collect();
          }
  
          return collect($this->value)->map(function ($value, $key) {
              return $this->notAllowed($key, $value);
          })->flatMap(function (Collection $fields, $type) {
              return $fields->map(function ($field) use ($type) {
                  return "{$type}.{$field}";
              });
          });
      }
docs: Get the fields that are invalid.
  
  @return Collection