Fedex API Ship : Error 400 when downloading label PDF via URL

I use the Fedex API to generate shipping labels (https://developer.fedex.com/api/fr-fr/catalog/ship/v1/docs.html) but I can't download the label via the url provided.

Here is the URL :

https://wwwtest.fedex.com/document/v1/cache/retrieve/SH,f15d003851096ec9794972069960_SHIPPING_Z?isLabel=true&autoPrint=false&retrievalMode=THERMAL

Here is the response :

{"transactionId": "94bb8cb1-e6d2-430a-a100-9850a57513b2","errors": [        {"code": "SHIPPING.DOCUMENT.NO.BYTES","message": "We are unable to process this request. Please try again later or contact FedEx Customer Service."        }    ]}

<?php$url = "https://wwwtest.fedex.com/document/v1/cache/retrieve/SH,f15d003851096ec9794972069960_SHIPPING_Z?isLabel=true&autoPrint=false&retrievalMode=THERMAL";$arrContextOptions=array("ssl"=>array("verify_peer"=>false,"verify_peer_name"=>false,        ),    );$response = file_get_contents($url, false, stream_context_create($arrContextOptions));?>