Getting error while inserting data to mysql database

While Inserting data to mysql database i'm getting the error coming below, and as of some previous question's answer on stackoverflow i put protected $fillable in my model file but still showing error.

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`app`.`articles`, CONSTRAINT `articles_ibfk_1` FOREIGN KEY (`article_id`) REFERENCES `users` (`id`)) (SQL: insert into `articles` (`article_head`, `article_body`) values (abkskfbasfbv, zbfvaksdvaskdvjsdc
))

Article.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Article extends Model
{
  protected $fillable = [
      'article_head', 'article_body',
  ];
    public $timestamps = false;
}

migration file

<?php

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

class CreateArticlesTable extends Migration {

    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('articles', function(Blueprint $table)
        {
            $table->increments('article_id');
            $table->string('article_head', 500);
            $table->string('article_body', 10000);
            $table->dateTime('created_on');
            $table->timestamps();
        });
    }


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

}

2016_04_27_181606_add_foreign_keys_to_articles_table.php

<?php

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

class AddForeignKeysToArticlesTable extends Migration {

    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::table('articles', function(Blueprint $table)
        {
            $table->foreign('article_id', 'articles_ibfk_1')->references('id')->on('users')->onUpdate('RESTRICT')->onDelete('RESTRICT');
        });
    }


    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::table('articles', function(Blueprint $table)
        {
            $table->dropForeign('articles_ibfk_1');
        });
    }

}

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