Coupon Magento API Soap

I have a problem with the Magento Coupon API when I do:

$couponCode = "test";

$resultCartCoupon = $proxy->call($sessionId, "cart_coupon.add", array($shoppingCartId, $couponCode));

I always have:

  

Uncaught SoapFault exception: [1083] Coupon is not valid

If I try the coupon code on the front-end , it's okay. Is there anyone who has used this part of the API with success?