JSONSerialization swift response is always nil

Question

I'm performing a post request over my rest api that I built in node. Then the data are stored in a mongodb collection.

This is my code that I use to post the request:

    // create dataTask using the session object to send data to the server
let task = session.dataTask(with: request) { data, response, error in

    if let error = error {
        print("Post Request Error: \(error.localizedDescription)")
        return
    }

    // ensure there is valid response code returned from this HTTP response
    guard let httpResponse = response as? HTTPURLResponse,
        (200...299).contains(httpResponse.statusCode)
    else {
        print("Invalid Response received from the server")
        return
    }

    // ensure there is data returned
    guard let responseData = data else {
        print("nil Data received from the server")
        return
    }

    do {
        // create json object from data or use JSONDecoder to convert to Model stuct
        if let jsonResponse = try JSONSerialization.jsonObject(with: responseData, options: .mutableContainers) as? [String: ErrorHandler] {
            print(jsonResponse)
            DispatchQueue.main.async {
                self?.isLoading = false
                self?.signedIn = true
            }
        } else {
            print("data maybe corrupted or in wrong format")
            throw URLError(.badServerResponse)
        }
    } catch let error {
        print(error.localizedDescription)
    }
}
task.resume()

The problem is that the responseJson is always nil. I tried to perform the same request with postman and I get the response correctly. What is the problem? Also because the data are correctly uploaded everywhere.

This is my postman result of the same post request.