Can not create a unique index on a column of an existing table

[ngmy]

Package	Version
laravel/framework	5.1.43 (LTS)
rafis/schema-extended	1.0.1
MySQL	5.5.46

I can not create a unique index on a column of an existing table.

I have two migrations files.

The first one is the 2016_08_29_135919_create_users_table.php file:

<?php

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateUsersTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('users', function (Blueprint $table) {
            $table->increments('id');
            $table->string('email');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('users');
    }
}

The second is the 2016_08_29_140747_add_index_to_users_table_email_column.php file:

<?php

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class AddIndexToUsersTableEmailColumn extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::table('users', function (Blueprint $table) {
            $table->unique('email');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::table('users', function (Blueprint $table) {
            $table->dropUnique('users_email_unique');
        });
    }
}

When I using SchemaExtended\Schema and executing the Artisan migrate command, the following error occur:

$ php artisan migrate


  [Illuminate\Database\QueryException]                                                                                                                                  
  SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the  
   right syntax to use near ')' at line 1 (SQL: alter table `users` add unique users_email_unique())                                                                    



  [PDOException]                                                                                                                                                        
  SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the  
   right syntax to use near ')' at line 1                                                                                                                               

The second migration seem is not applied.

mysql> desc users;
+------------+------------------+------+-----+---------------------+----------------+
| Field      | Type             | Null | Key | Default             | Extra          |
+------------+------------------+------+-----+---------------------+----------------+
| id         | int(10) unsigned | NO   | PRI | NULL                | auto_increment |
| email      | varchar(255)     | NO   |     | NULL                |                |
| created_at | timestamp        | NO   |     | 0000-00-00 00:00:00 |                |
| updated_at | timestamp        | NO   |     | 0000-00-00 00:00:00 |                |
+------------+------------------+------+-----+---------------------+----------------+
4 rows in set (0.00 sec)

When I using Illuminate\Support\Facades\Schema and executing the Artisan migrate command, the error does not occur:

$ php artisan migrate
Migrated: 2016_08_29_135919_create_users_table
Migrated: 2016_08_29_140747_add_index_to_users_table_email_column

Two of the migration has been applied:

mysql> desc users;
+------------+------------------+------+-----+---------------------+----------------+
| Field      | Type             | Null | Key | Default             | Extra          |
+------------+------------------+------+-----+---------------------+----------------+
| id         | int(10) unsigned | NO   | PRI | NULL                | auto_increment |
| email      | varchar(255)     | NO   | UNI | NULL                |                |
| created_at | timestamp        | NO   |     | 0000-00-00 00:00:00 |                |
| updated_at | timestamp        | NO   |     | 0000-00-00 00:00:00 |                |
+------------+------------------+------+-----+---------------------+----------------+
4 rows in set (0.06 sec)