Laravel problem with pivot table

I have two tables: one for travel and another for clients. In the pivot table it relates to each other in ManyToMany.

In the migration of the pivot table I have the following:

<?php

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

class CreateCustomerTourTable extends Migration
{
    public function up()
    {
        Schema::create('customer_tour', function (Blueprint $table) {
            $table->increments('id');
            /** Satisfacción */
            $table->char('satisfaccion', 1);
            /** Los campos de fechas */
            $table->timestamps();
            $table->softDeletes();
            /** La definición de los campos que se usarán como claves foráneas */
            $table->integer('customer_id')->unsigned();
            $table->integer('tour_id')->unsigned();
            /** La declaración de las claves foráneas en los campos necesarios. */
            $table->foreign('customer_id')->references('id')->on('customers');
            $table->foreign('tour_id')->references('id')->on('tours');
        });
    }

    public function down()
    {
        Schema::dropIfExists('customer_tour');
    }
}

What matters to me is the satisfaction field, where the degree of customer satisfaction with the trip will be recorded. In the factory I have the following:

<?php

use Faker\Generator as Faker;

$factory->define(App\CustomerTour::class, function (Faker $faker) {
    return [
        'satisfaccion' => $faker->randomElement(['A', 'B', 'C'])
    ];
});

In the Tour.php model (that of the trips) I have added the following (only the customers method):

public function customers()
{
    return $this->belongsToMany(Customer::class)->withTimeStamps()->using('App\CustomerTour')->withPivot('satisfaccion');
}

In the client model ( Customer.php ) I have the method tours asi;

public function tours()
{
    $this->belongsToMany(Tour::class)->withTimeStamps()->using('App\CustomerTour')->withPivot('satisfaccion');
}

The model CustomerTour.php (that of the pivot table) is as follows:

<?php

namespace App;

use Illuminate\Database\Eloquent\Relations\Pivot;
use Illuminate\Database\Eloquent\SoftDeletes;

class CustomerTour extends Pivot
{
    use SoftDeletes;
    protected $table = 'customers';
    protected $fillable = [
        'satisfaccion',
        'created_at',
        'updated_at'
    ];
    protected $attributes = [
        'satisfaccion'
    ];
    protected $guarded = [
        'id',
        'created_at',
        'updated_at'
    ];
    public $timestamps = true;
    protected $dates = [
        'deleted_at'
    ];
}

Finally, the DatabaseSeeder.php remains like this:

<?php

use App\Operator;
use App\Tour;
use App\Customer;
use App\CustomerTour;
use Illuminate\Database\Seeder;

class DatabaseSeeder extends Seeder
{
    public function run()
    {
        factory (Operator::class, 20)->create();
        $customers = factory (Customer::class, 300)->create();
        factory (Tour::class, 100)
        ->create()
            ->each(function ($tour) use ($customers)
                {
                    $tour->customers()
                        ->attach($customers
                            ->random(mt_rand(10, 40))
                            ->pluck('id')
                        );
                }
            );
    }
}

Everything works fine, except the field satisfaccion of the pivot table, which I filled in to NULL , instead of a randomElement , as I indicated in the factory.

Why does this happen? How can I fix it?

Thank you all.