Coupon Magento API Soap

Question

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 front-end, no problem.
Is there anyone who has used that part of the API successfully?