Cannot read property 'toString' of undefined when deletting a carholder

Describe the bug
a Cannot read property 'toString' of undefined error is respone when deletting a carholder,
target Employee satisfies both unassign and zero amount，but the issue this error still exist.
How should I solve this problem?

obj = {
"employee_number": null,
"balance": "0",
"is_admin": false,
"groups": [],
"_id": "6243e305b8b32b1dcf029613",
"company_id": "612d122665d86c75a41de6e4",
"user_id": {
"address": {
"country": "US",
"line_1": "4833 RUGBY AVE",
"city": "BETHESDA",
"state": "MD",
"zip_code": "20814",
"phone": "4242969622"
},
"first_login": false,
"_id": "6243e304b8b32b837802960d",
"first_name": "Delete",
"last_name": "Der",
"email": "deteter@gamil.com",
"avatar_link": "https://api.thepennyinc.com/users/581a9c7cb98f055656962a10/avatar"
},
"gpa": {
"currency_code": "USD",
"ledger_balance": 0,
"available_balance": "0",
"credit_balance": 0,
"pending_credits": 0
},
"created_date": "2022-03-30T04:56:37.750Z"
}

Additional context
this is my demo Api progarme in python:

import re
import time
import traceback
import requests
import json
import names
import string
import random
from lxml import etree
from concurrent.futures import ThreadPoolExecutor

ProductApi = 'https://api-prod.thepennyinc.com/api/v1/'

X_PENNY_API_KEY= 'My Api Key'
company_id = 'My company_id'

headers = {
'X-PENNY-API-KEY': X_PENNY_API_KEY
}

def deleteCarHolder():

Api = ProductApi+"employees/" + obj['_id'] --> 6243e305b8b32b1dcf029613

response = requests.delete(Api, headers=headers)

print(response.text) --> {"success":false,"result":{"message":"Cannot read property 'toString' of undefined"}}

test = deleteCarHolder()
test = {"success":false,"result":{"message":"Cannot read property 'toString' of undefined"}}

The report names no file, test, or entry point in postman-code-generators; start by checking whether the DELETE request is handled by this repository or by the external API URL. Reproduce the request with the supplied employee payload and determine which missing value causes the server error. Done means a confirmed repository fix or a clear routing and ownership conclusion.