Declaration of App\Http\Controllers\Controller::formatValidationErrors()

The Document clearly says, we need to use "Illuminate\Contracts\Validation\Validator" in page http://lumen.laravel.com/docs/validation

but when I try, at 5.1.5 version of lumen,

namespace App\Http\Controllers;

use Illuminate\Contracts\Validation\Validator;
//use Illuminate\Validation\Validator;
use Laravel\Lumen\Routing\Controller as BaseController;

abstract class Controller extends BaseController {

    /**
         * {@inheritdoc}
    */
  protected function formatValidationErrors(Validator $validator)
  {
     return $validator->errors()->all();
  }

}

following error appears

ErrorException in Controller.php line 8:
Declaration of App\Http\Controllers\Controller::formatValidationErrors() should be compatible with Laravel\Lumen\Routing\Controller::formatValidationErrors(Illuminate\Validation\Validator $validator)

It works when I comment the

use Illuminate\Contracts\Validation\Validator;

and un-comment the

//use Illuminate\Validation\Validator;

what is going on?