Laravel Request Validation: The given data was invalid

Question

PostRequest.php:

<?php

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;

class PostStoreRequest extends FormRequest
{
    public function authorize()
    {
        return true;
    }

    public function rules()
    {
        return [
            'title' => 'required',
            'content' => 'required',
        ];
    }

    public function messages()
    {
        return [
            'title.required' => '文章标题不能为空',
            'content.required'  => '文章内容不能为空',
        ];
    }

}

PostController.php:

class PostController extends Controller
{
    public function store(PostStoreRequest $request)
    {
        // ...
    }
}

当我验证通过时，一切正常。
但是验证不通过时，返回的信息不是 PostStoreRequest.php 中 messages 函数中定义的错误提示信息，而是：

{
    "message": "The given data was invalid.",
    "status_code": 500,
    "debug": {
        "line": 117,
        "file": "/var/www/site/vendor/laravel/framework/src/Illuminate/Foundation/Http/FormRequest.php",
        "class": "Illuminate\\Validation\\ValidationException",
        "trace": [
            // ...
        ]
    }
}

不知道哪里出错了。