Error is raised on seeding with index defined

In laravel 5.8 app I created migration file with 1 field and 1 index added

class SubscriptionsTableAddIsFreeField extends Migration
{
    public function up()
    {
        Schema::table('subscriptions', function (Blueprint $table) {
            $table->boolean('is_free')->default(false)->after('source_service_subscription_id');
            $table->index([ 'user_id', 'is_free' ], 'subscriptions_user_id_is_free_index');

        });
    }

    public function down()
    {
        Schema::table('subscriptions', function (Blueprint $table) {
            $table->dropForeign('subscriptions_user_id_is_free_index');
            $table->dropColumn('is_free');
        });
    }

But running command

php artisan migrate:refresh --seed

I got error :

: SQLSTATE[42000]: Syntax error or access violation: 1091 Can't DROP 'subscriptions_user_id_is_free_index'; check that column/key exists (SQL: alter table `vt2_subscriptions` drop foreign key `subscriptions_user_id_is_free_index`)

  at /mnt/_work_sdb8/wwwroot/lar/votes/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   Doctrine\DBAL\Driver\PDOException::("SQLSTATE[42000]: Syntax error or access violation: 1091 Can't DROP 'subscriptions_user_id_is_free_index'; check that column/key exists")
      /mnt/_work_sdb8/wwwroot/lar/votes/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:119

  2   PDOException::("SQLSTATE[42000]: Syntax error or access violation: 1091 Can't DROP 'subscriptions_user_id_is_free_index'; check that column/key exists")
      /mnt/_work_sdb8/wwwroot/lar/votes/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:117

  Please use the argument -v to see more details.

What is wrong in defintion of my migration?

I know that laravel migration has methods

if (Schema::hasTable('users')) {
    //
}

if (Schema::hasColumn('users', 'email')) {
    //
}

But can I check indexes?

via Chebli Mohamed