TikTok API - Troubleshooting "Signature is Invalid" Error in "Get Order Settlements" API Call with Node.js

Question

I'm working on integrating the TikTok API in a Node.js application and facing an issue with the "Get Order Settlements" endpoint. Despite following the API documentation and successfully implementing other endpoints, this particular call keeps returning a "signature is invalid" error.

Environment:

Node.js

Express.js

MongoDB

Problematic Endpoint: /api/finance/order/settlements

Issue: The API call to get order settlements consistently fails with the error message: "Error: signature is invalid". I'm using a custom function generateSignature for creating HMAC-SHA256 signatures.

Here's my generateSignature function:

function generateSignature(path, params, secret) {
  const sortedKeys = Object.keys(params).sort();
  let input = path;
  sortedKeys.forEach(key => {
      input += key + params[key];
  });
  input = secret + input + secret;
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(input);
  return hmac.digest('hex');
}

Express.js Controller Function:

// Simplified version for brevity
exports.getTikTokOrderSettlements = async (req, res) => {
    // ... [setup and validation code]

    const queryParams = {
        app_key: appKey,
        timestamp: timestamp,
        access_token: accessToken,
        shop_id: shopId,
        order_id: order_id,
    };

    const sign = generateSignature(path, queryParams, secret);

    queryParams.sign = sign;
    const queryString = new URLSearchParams(queryParams).toString();
    const tiktokApiUrl = `https://open-api.tiktokglobalshop.com${path}?${queryString}`;

    // ... [API call using fetch]
};

This method has worked for other endpoints (e.g., getting authorized shops), but not for the order settlements endpoint. The error persists even after double-checking the API key, secret, and other parameters.

What I've Tried:

• Ensuring all relevant query parameters are included in the signature.
• Checking parameter order and encoding.
• Synchronizing timestamps to GMT (UTC+00:00).
• Using hardcoded values from a known working example (from TikTok's API testing tool).

I'm at a loss as to why this specific endpoint is failing. Is there something I'm missing in the signature generation process, or is there a known issue with this TikTok API endpoint?

Any insights or suggestions would be greatly appreciated.