Laravel 5.1 - Paypal error

i have a problem with complete payment with paypal, i have this error:

Got Http response code 400 when accessing http://ift.tt/1f12BzJ.

MY CONTROLLER

$payer = new Payer();
        $payer->setPaymentMethod('paypal');

        $items = array();
        $subtotal = 0;
        $cart = \Session::get('cart'); // Get info by my session cart
        $currency = 'EUR';

        // i extract info by session cart and count the Subtotal 

        foreach($cart as $producto){
            $item = new Item();
            $item->setName($producto->name)
            ->setCurrency($currency)
            ->setDescription($producto->extract)
            ->setQuantity($producto->quantity)
            ->setPrice($producto->price);

            $items[] = $item;
            $subtotal += $producto->quantity * $producto->price;
        }

        // i calculated my $price_coupon and here i want make the discount.
        // price coupon can be ZERO or >0

        $subtotal_coupon = $subtotal - $price_coupon  ;

        $item_list = new ItemList();
        $item_list->setItems($items);
        // set up subtotal (with discount coupon) 
        $details = new Details();
        $details->setSubtotal($subtotal_coupon)
        ->setShipping($get_session->shipping);

        // here i calculated the total amount for paypal payment
        // i extracted the price of shipping from my DB, so $get_session->shipping work well

        $total = $subtotal_coupon + $get_session->shipping;

        $amount = new Amount();
        $amount->setCurrency($currency)
            ->setTotal($total)
            ->setDetails($details);

Important! if i don't add my coupon, it work well! like this:

    $payer = new Payer();
    $payer->setPaymentMethod('paypal');

    $items = array();
    $subtotal = 0;
    $cart = \Session::get('cart'); //get session cart
    $currency = 'EUR';

    // extract info by session cart
    foreach($cart as $producto){
        $item = new Item();
        $item->setName($producto->name)
        ->setCurrency($currency)
        ->setDescription($producto->extract)
        ->setQuantity($producto->quantity)
        ->setPrice($producto->price);

        $items[] = $item;
        $subtotal += $producto->quantity * $producto->price;
    }

    // items
    $item_list = new ItemList();
    $item_list->setItems($items);

    $details = new Details();
    $details->setSubtotal($subtotal)
    ->setShipping($get_session->shipping);

    $total = $subtotal + $get_session->shipping;

    $amount = new Amount();
    $amount->setCurrency($currency)
        ->setTotal($total)
        ->setDetails($details);

I don't know why when i add the calculation of discount COUPON, it give me a error.

Thank you for your help guys!

via Chebli Mohamed