errno: 150 "Foreign key constraint is incorrectly formed"

I have many to one relation from Category to Post, and following are from respective migrations:

Posts: public function up() { Schema::create('posts', function (Blueprint $table) {

        $table->increments('id');

        $table->string('title');

        $table->string('body')->nullable();

        $table->integer('cat_id')->unique()->nullable();

        $table->foreign('cat_id')
            ->references('id')->on('post_categories')
            ->onDelete('cascade');

        $table->dateTime('event_at')->nullable();

        $table->timestamps();
    });

Post Categories:

public function up() { Schema::create('post_categories', function (Blueprint $table) {

        $table->increments('id')->unsigned()->unique();

        $table->string('cat_name');

        $table->timestamps();
    });
}

}

Models: Post: class Post extends Model {

use SearchableTrait;

protected $searchable = [

    'columns' => [
        'posts.title' => 10,
        'posts.body' => 5,
    ]

];

public function postcategory() {

    return $this->belongsTo(PostCategory::class);

}

}

Post_Category: class PostCategory extends Model { public function posts() {

    return $this->hasMany(Post::class);

}

}

I keep getting this error message when i try migrating(php artisan migrate):

[Illuminate\Database\QueryException] SQLSTATE[HY000]: General error: 1005 Can't create table artgallery.#sql-36c4_160 (errno: 150 "Foreign key constraint is incorrectly formed") (SQL: alter table posts add constraint pos ts_cat_id_foreign foreign key (cat_id) references post_categories (id) on delete cascade)

[PDOException] SQLSTATE[HY000]: General error: 1005 Can't create table artgallery.#sql-36c4_160 (errno: 150 "Foreign key constraint is incorrectly formed")

Please guide where I could be committing mistake and what is the solution.