Error when generating label via shipment

I'm trying to use the sandbox and test generating a label at the same time as a shipment, however when I query the shipment after creation and get the label I get the error:

"last_failure_http_response_messages": [
{
"code": "NoMethodError",
"content": "undefined method `[]' for nil:NilClass"
}
],

Could you have a look and let me know what the call is missing/the cause? Thanks!