how do create 2 forign key with laravel migration

i have letter's model and this model has 2 forign key from attachments and contacts

public function up()
{
    Schema::create('letters', function (Blueprint $table) {
        $table->increments('id');
        $table->integer('contact_id')->unsigned();
        $table->integer('attachment_id')->unsigned();
        $table->timestamps();
        $table->foreign('contact_id')->references('id')->on('contacts');
        $table->foreign('attachment_id')->references('id')->on('attachments');
    });
}

i do this. but when i type artisan migration SQL has error

General error: 1005 Can't create table ss.#sql-1064_4a (errno: 150 "Foreign
key constraint is incorrectly formed") (SQL: alter table letters add constraint letters_attach
ment_id_foreign foreign key (attachment_id) references attachments (id))

via Chebli Mohamed