一般錯誤：laravel 遷移 php 中的 1215

這是我的錯誤：

General error: 1215 Cannot add foreign key constraint (SQL: alter table `author_book` add constraint `author_book_author_id_foreign` foreign key (`author_id`) references `author` (`id`))

我有表格書籍和帶有數據透視表的作者：

        Schema::create('books', function (Blueprint $table) {
        $table->increments('id');
        $table->unsignedInteger('user_id');
        $table->string('name');
        $table->unsignedInteger('pages');
        $table->string('ISBN');
        $table->integer('price');
        $table->date('published_at');

        $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade')->onUpdate('cascade');

作者表：

    public function up()
{
    Schema::create('authors', function (Blueprint $table) {
        $table->increments('id');
        $table->string('author');
        $table->date('birthdate');
        $table->timestamps();
    });
}

和我的數據透視表：

            $table->increments('id');

        $table->unsignedInteger('author_id');
        $table->unsignedInteger('book_id');

        $table->foreign('author_id')->references('id')->on('author');
        $table->foreign('book_id')->references('id')->on('books');
        $table->timestamps();

請幫我找出錯誤！