Integrating APP of Laravel with Mercadopago

I'm having a problem when creating my payment preference, such a problem is a

  

"Params Error" 400.

Form with which I send the products of the cart to my controller:

<form class="form-horizontal" action="{!! route('payment.process') !!}" method="post">
  {{ csrf_field() }}
  <input type="hidden" name="ctoken" id="ctoken" value="{!! $cart_token !!}">
  <input type="submit" name="pagar" value="Pagar" class="btn btn-success btn-block btn-sm">
</form>

Controller in question:

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Http\Requests;
use App\Models\Cart;
use Exception;
use MP;

class PaymentController extends Controller
{
  /**
   * Display a listing of the resource.
   *
   * @return Response
   */
  public function process(Request $request)
  {

    $mp = new MP (env('OCULTO'), env('OCULTO'));

    $user = auth()->user();

    $preferenceData = [
        'external_reference' => $request->ctoken,
        'payer'              => [
            'name' => $user->name,
            'email' => $user->email,
            'date_created' => $user->created_at
        ]
    ];

    $entries = Cart::where('session_id', '=', $request->ctoken)->get();
    foreach ($entries as $e):
        $preferenceData['items'][] = [
            'id'          => $e->product_id,
            'category_id' => 'zapato',
            'title'       => $e->product_name,
            'description' => 'esta es la descripción del zapato',
            'picture_url' => 'https://placehold.it/600x600',
            'quantity'    => $e->qty,
            'currency_id' => 'VEF',
            'unit_price'  => $e->price,
        ];
    endforeach;

    $preference = $mp->create_preference($preferenceData);

    // return init point to be redirected
    //return $preference['response']['init_point'];
    return dd($preferenceData);

  }
}

In the hidden parameters I have the credentials provided by mercadopago.