Не получается пост запрос к fastAPI

Не получается запрос пост к fast Api Сервер:

import httpx
import requests
from pydantic import BaseModel
from fastapi import FastAPI
from typing import Union
from fastapi.middleware.cors import CORSMiddleware

class ChipsAndRules(BaseModel):
    chips: str
    rules: Union[str, None] = None
    lang: Union[str, None]  = None

@app.post("/newChips/")
async def create_new_chips(old_chips: ChipsAndRules):
    return {"chips and rules " : old_chips.chips+' '+old_chips.rules}

Вот вызов:

req = new XMLHttpRequest();  
req.onreadystatechange = function (e) {
  if (req.readyState == 4) {
      console.log( 'Проверка, ответа' + req.responseText );       
  }
};
reqBody = {"chips" : "chip1",
            "rules": "rule1",
          };
req.open("POST", "http://ххх:9091/newChips/",true);
req.send(reqBody);

Получаю ответ:

422 (Unprocessable Entity)
{"detail":[{"type":"model_attributes_type","loc":["body"],"msg":"Input should be a valid dictionary or object to extract fields from","input":"[object Object]"

Не пойму, что не так.

upd. не хватало

req.setRequestHeader('Content-Type', 'application/json; charset=utf-8');
req.send(JSON.stringify(reqBody));