title: TypeError: action$.ofType(...).mergeMap is not a function
body: <p>I am new to reactjs and trying to integrate redux with my existing project.</p>
  
  <p>This is my <strong><code>index.js</code></strong> file in store</p>
  
  <pre><code>import 'rxjs'
  import { createStore, combineReducers, applyMiddleware } from 'redux'
  import { reducer as formReducer } from 'redux-form'
  import thunk from 'redux-thunk'
  import promise from 'redux-promise-middleware'
  import { createEpicMiddleware, combineEpics } from 'redux-observable'
  
  import user, { userEpic } from './user/duck'
  import app from './app'
  
  // Bundling Epics
  const rootEpic = combineEpics(
      userEpic
  )
  
  // Creating Bundled Epic
  const epicMiddleware = createEpicMiddleware(rootEpic)
  
  // Define Middleware
  const middleware = [
    thunk,
    promise(),
    epicMiddleware
  ]
  
  // Define Reducers
  const reducers = combineReducers({
    app,
    user,
    form: formReducer
  })
  
  // Create Store
  export default createStore(reducers,window.__REDUX_DEVTOOLS_EXTENSION__ &amp;&amp; window.__REDUX_DEVTOOLS_EXTENSION__(), applyMiddleware(...middleware))
  </code></pre>
  
  <p>And here is <strong><code>duck.js</code></strong></p>
  
  <pre><code>const createUserEpic = (action$) =&gt;
    action$
    .ofType(SIGNUP_CONCIERGE)
    .mergeMap((action) =&gt; {
      return Rx.Observable.fromPromise(api.signUpConcierge(action.payload))
      .flatMap((payload) =&gt; ([{
        type: SIGNUP_CONCIERGE_SUCCESS,
        payload
      }]))
      .catch((error) =&gt; Rx.Observable.of({
        type: SIGNUP_CONCIERGE_ERROR,
        payload: { error }
      }))
    })
  
  export const userEpic = combineEpics(
    createUserEpic
  )
  </code></pre>
  
  <p>which throws me error <code>TypeError: action$.ofType(...).mergeMap is not a function</code></p>
  
  <p>I have been getting this error since I updated react, react-redux, redux-observable versions.</p>
  
  <p>What I am doing wrong here? Please help!!!</p>