migrate:refresh showing error

2017_09_08_103048_create_students_table.php

<?php

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

class CreateStudentsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('students', function (Blueprint $table) {
            $table->increments('id');
            $table->string('fname');
            $table->string('mname');
            $table->string('lname');
            $table->string('dob');
            $table->string('gender');
            $table->string('bloodgroup');
            $table->string('bplace');
            $table->string('nationality');
            $table->string('religion');
            $table->mediumText('presentadd');
            $table->mediumText('permanentadd');
            $table->string('city');
            $table->string('ps');
            $table->string('zipcode');
            $table->string('phone');
            $table->string('mobile');
            $table->string('email')->unique();
            $table->timestamps();
        });
    }

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

2017_09_09_055201_add_rcs_to_students_table.php

<?php

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

class AddRcsToStudentsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::table('students', function ($table) {
            $table->string('roll')->unique()->after('email');
            $table->string('class')->after('roll');
            $table->string('section')->after('class');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::table('users', function ($table) {
            $table->dropColumn('roll');
            $table->dropColumn('class');
            $table->dropColumn('section');
        });
    }
}

error

The Following errors are showing  after php artisan:reset or  php artisan:refresh or php artisan:rollback command

[Illuminate\Database\QueryException]
SQLSTATE[42000]: Syntax error or access violation: 1091 Can't DROP 'roll'; check that column/key exists (SQL: alter table `users` drop `roll`)

[PDOException]                                                                                         
SQLSTATE[42000]: Syntax error or access violation: 1091 Can't DROP 'roll'; check that column/key exists

Error is Schema::table('students', function ($table)


Schema::table('users', function ($table) { $table->string('roll')->unique()->after('email'); $table->string('class')->after('roll'); $table->string('section')->after('class'); });