laravel5.7 外部キーエラー　SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint

「Laravelで読書記録システムを作る」に取り組んでいます
（http://learninglaravel.fuwafuwatask.com/migration/）

2018_07_08_070043_create_reading_records_table.php

のマイグレーションをかけたところ、タイトルのようなエラーがでました。

原因はどこにありますでしょうか？
よろしくお願いします

発生している問題・エラーメッセージ

Illuminate\Database\QueryException : SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table reading_records add constraint reading_records_reader_id_foreign foreign key (reader_id) references readers (id) on delete cascade)

at /Applications/MAMP/htdocs/board/vendor/laravel/framework/src/Illuminate/Database/Connection.php:664
660| // If an exception occurs when attempting to run a query, we'll format the error
661| // message to include the bindings with SQL, which will make this exception a
662| // lot more helpful to the developer instead of just the database's errors.
663| catch (Exception $e) {

664| throw new QueryException(

665|                 $query, $this->prepareBindings($bindings), $e
666|             );
667|         }
668|

Exception trace:

1 PDOException::("SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint")
/Applications/MAMP/htdocs/board/vendor/laravel/framework/src/Illuminate/Database/Connection.php:458

2 PDOStatement::execute()
/Applications/MAMP/htdocs/board/vendor/laravel/framework/src/Illuminate/Database/Connection.php:458

Please use the argument -v to see more details.

該当のソースコード

PHP
1<?php
2
3use Illuminate\Support\Facades\Schema;
4use Illuminate\Database\Schema\Blueprint;
5use Illuminate\Database\Migrations\Migration;
6
7class CreateReadingRecordsTable extends Migration
8{
9    /**
10     * Run the migrations.
11     *
12     * @return void
13     */
14    public function up()
15    {
16        Schema::create('reading_records', function (Blueprint $table) {
17            $table->engine = 'InnoDB';
18            $table->increments('id');
19            $table->integer('book_id')->unsigned();
20            $table->integer('reader_id')->unsigned();
21            $table->year('year_read');
22            $table->tinyInteger('month_read');
23            $table->string('report');
24            $table->timestamps();
25            $table->foreign('book_id')
26            ->references('id')
27            ->on('books')
28            ->onDelete('cascade');
29            $table->foreign('reader_id')
30            ->references('id')
31            ->on('readers')
32            ->onDelete('cascade');
33        });
34    }
35
36    /**
37     * Reverse the migrations.
38     *
39     * @return void
40     */
41    public function down()
42    {
43        Schema::dropIfExists('reading_records');
44    }
45}
46