Can't add foreign key in migration.

I have one migration to execute. And here is my code:

public function up()
    {
        Schema::table('students', function (Blueprint $table) {
            //
            $table->foreign('phone_id')->reference('id')->on('phone');
        });
    }

where i execute migration, it said i have an error in my sql. And i run php artisan migrate --pretend to output the sql:

alter table `students` add constraint `students_phone_id_foreign` foreign key (`phone_id`) references `phone` ()

As you can see, there is a () at the end of the sql. How does it come out?