Testing iOS auto renewable subscriptions with promocodes

Question

I've just developed an iOS app with auto-renewable susbscriptions. I've tested them deeply with sandbox test users without problems.

Now the app has been aproved by Apple and I've tested with a friend promocodes for this auto-renewable subscription.

My friend has just redeemed the code without any problem but when he tries to access the content he receives an internal app error. This error is generated when the app refresh the receipt and tries to parse it's content, to be exact the error is generated here:

guard let receipts_array = json["latest_receipt_info"] as? [Dictionary<String, Any>] else {
            // throw error
            return
        }

I assumed that auto-renewal promocodes worked the same way than a purchase, so the flow and the receipt should be equal with a promocode and with a purchase. Am I wrong? Does this error mean that the receipt has not the latest_receipt_info field? Should I search for the promocode info in any other field of the receipt?

Thank you very much!