SQLSTATE[23000]: Integrity constraint violation during migration in Laravel 4 - mysql

Question

I have a problem while migrating
Migration File
<?php
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;
class AlterTableMm extends Migration {
/**
* Run the migrations.
*
* #return void
*/
public function up()
{
Schema::create('downloads', function($table){
$table->engine = 'InnoDB';
$table->increments('id')->unsigned();
$table->string('title',255);
$table->string('count',45)->default(0);
$table->timestamps();
$table->integer('user_id')->unsigned();
$table->foreign('user_id')->references('id')->on('users');
});
Schema::table('marketing_materials', function($table)
{
$table->string('download_total', 45 )->default(0);
$table->integer('download_id')->unsigned();
$table->foreign('download_id')->references('id')->on('downloads');
});
}
/**
* Reverse the migrations.
*
* #return void
*/
public function down()
{
Schema::table('users', function($table)
{
$table->dropForeign('users_user_id_foreign');
});
Schema::drop('downloads');
Schema::table('marketing_materials', function($table)
{
$table->dropForeign('marketing_materials_download_id_foreign');
});
Schema::table('marketing_materials', function($table)
{
$table->dropColumn('download_total');
$table->dropColumn('download_id');
});
}
}
Error Message
[Illuminate\Database\QueryException]
SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`distributor-local`.`#sql-668_3d`, CONSTRAINT `marketing_mater
ials_download_id_foreign` FOREIGN KEY (`download_id`) REFERENCES `downloads` (`id`)) (SQL: alter table `marketing_materials` add constraint marketing_materials_download_id_foreign fo
reign key (`download_id`) references `downloads` (`id`))
[PDOException]
SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`distributor-local`.`#sql-668_3d`, CONSTRAINT `marketing_mater
ials_download_id_foreign` FOREIGN KEY (`download_id`) REFERENCES `downloads` (`id`))
I have all the required tables
users
marketing_materials
I just recently migrate my downloads table
Can someone tell me what did I do wrong ?
The part that confuse me the most of that error message is that, all the contents are added the database fine. See the picture below
I also alter my marketing_materials table, and that's also work.
Should I just ignore the error message ???