为什么DocuSign CustomFields不能从信封反序列化？

import requests
import json

# 设置API端点和认证信息
api_endpoint = "https://api.docusign.net/restapi/v2.1/accounts/{accountId}/envelopes"
access_token = "your_access_token"

# 创建信封请求体
envelope_definition = {
    "emailSubject": "Please sign this document",
    "documents": [
        {
            "documentBase64": "base64_encoded_document_content",
            "name": "Document.pdf",
            "fileExtension": "pdf",
            "documentId": "1"
        }
    ],
    "recipients": {
        "signers": [
            {
                "email": "signer@example.com",
                "name": "John Doe",
                "recipientId": "1",
                "tabs": {
                    "customFields": [
                        {
                            "name": "Field1",
                            "type": "Text",
                            "value": "Sample Text"
                        }
                    ]
                }
            }
        ]
    },
    "status": "sent"
}

# 发送请求
headers = {
    "Authorization": f"Bearer {access_token}",
    "Content-Type": "application/json"
}
response = requests.post(api_endpoint, headers=headers, data=json.dumps(envelope_definition))

# 检查响应
if response.status_code == 201:
    print("Envelope created successfully")
else:
    print(f"Error creating envelope: {response.text}")