TypeError: You provided an invalid object where a stream was expected. Redux-observal Problem

TypeError: You Provided an invalid object where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.

//action.js

import { map,mergeMap } from 'rxjs/operators';
import {ofType } from 'redux-observable'
import { ajax } from 'rxjs/ajax';



const fetchUser = username => ({ type: 'FETCH_USER', payload: username });
export const fetchUserFulfilled = payload => ({ type: 'FETCH_USER_FULFILLED', payload });

export const fetchUserEpic = action$ => action$.pipe(
  ofType('FETCH_USER'),
  mergeMap(action =>
    ajax.getJSON(`https://api.github.com/users/${action.payload}`).pipe(
      map(response => fetchUserFulfilled(response))
    )
  )
);

export default fetchUser

Store

const epicMiddleware = createEpicMiddleware();
const store = createStore(rootReducer,applyMiddleware(epicMiddleware))

epicMiddleware.run(rootEpic);

I found the answer. It is not the problem of redux-observable.

I imported the epic instead of action creators and use it in the react component.

Thank you.