Error NOTARY_HOSTED_SIGNER_ID_REQUIRED when trying to send an eNotary envelope

Let my envelope be as below :

{
    "allowReassign": "false",
    "documents": [
        {
            "documentBase64": "JVBE",
            "documentId": "1",
            "fileExtension": "PDF",
            "name": "DocumentToNotarize"
        }
    ],
    "emailSubject": "Notary Test",
    "enableWetSign": "false",
    "notification": {
        "expirations": {
            "expireAfter": "4",
            "expireEnabled": "True"
        }
    },
    "recipients": {
        "inPersonSigners": [
            {
                "email": "signer@domain.com",
                "inPersonSigningType": "notary",
                "name": "Signer",
                "notaryHost": {
                    "deliveryMethod": "email",
                    "email": "notary@domain.com",
                    "name": "Notary",
                    "recipientId": "995a0019-f0bc-47bf-94d5-426607388f7b",
                    "tabs": {
                        "notarizeTabs": [
                            {
                                "documentId": "1",
                                "pageNumber": "1",
                                "xPosition": "100",
                                "yPosition": "100"
                            }
                        ]
                    }
                },
                "recipientId": "3fa6ffaf-f87e-4f27-9129-6d12d987f59b",
                "tabs": {
                    "signHereTabs": [
                        {
                            "documentId": "1",
                            "pageNumber": "1",
                            "recipientId": "3fa6ffaf-f87e-4f27-9129-6d12d987f59b",
                            "scaleValue": "0.6",
                            "xPosition": "45",
                            "yPosition": "527"
                        }
                    ]
                }
            }
        ]
    },
    "status": "sent"
}

Which triggers the following error :

{
    "errorCode": "NOTARY_HOSTED_SIGNER_ID_REQUIRED",
    "message": "The host signer Id is required to associate with notary in person signer."
}

I have found, surprisingly, that if I only change the signer's recipientId to an integer as below, it works !? Am I missing something ? GUIDs are allowed right ?

 "recipientId": "3",
        "tabs": {
          "signHereTabs": [
            {
              "documentId": "2",
              "pageNumber": "1",
              "recipientId": "3",
              "scaleValue": "0.6",
              "xPosition": "45",
              "yPosition": "527"
            }
          ]
        }
      }

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