title: Redux: Unexpected key found in preloadedState argument passed to createStore
body: <p>Can you help me with exception <code>Unexpected key "userName" found in preloadedState argument passed to createStore. Expected to find one of the known reducer keys instead: "default". Unexpected keys will be ignored.</code></p>
  
  <p>I discovered this <a href="https://stackoverflow.com/a/37998706/4443351">Link</a> but it doesn't help me. I don't undestand something, maybe this part from documentation: <strong>plain object with the same shape as the keys passed to it</strong></p>
  
  <p>Can you exlain me my mistake on my example?</p>
  
  <pre><code>import React from "react";
  import ReactDOM from "react-dom";
  import { Provider } from 'react-redux';
  import { createStore, combineReducers, applyMiddleware } from 'redux';
  import  App from './containers/App.jsx';
  import * as reducers from './reducers'
  import types from './constants/actions';
  
  const reducer = combineReducers(reducers);
  
  const destination = document.querySelector("#container");
  
  var store = createStore(reducer, {
      userName : ''
  });
  
  
  ReactDOM.render(
      &lt;Provider store={store}&gt;
          &lt;App/&gt;
      &lt;/Provider&gt;,
      destination
  );
  
  console.log(1)
  
  store.subscribe( ()=&gt; {
  console.log("-------------------------")
  let s = store.getState()
  console.log("STATE = " + s)
  for (var k in s) {
      if (s.hasOwnProperty(k)) {
          console.log("k = " + k + "; value = " + s[k]);
      }
  }
  })
  
  store.dispatch({
          type: types.LOAD_USER_NAME,
          userName : "Oppps1"
  })
  </code></pre>
  
  <p>my reducer:</p>
  
  <pre><code>import types from './../constants/actions'
  
  export default function userNameReducer (state = {userName : 'N/A'}, action) {
  console.log('inside reducer');
  switch (action.type) {
      case types.LOAD_USER_NAME:
          console.log("!!!!!!!!!!!!!!!")
          console.log("action.userName = " + action.userName)
          for (var k in state) {
              if (state.hasOwnProperty(k)) {
                  console.log("k = " + k + "; value = " + state[k]);
              }
          }
          return action.userName;
      default:
          return state
  }
  }
  </code></pre>
  
  <p>result in console after execution:</p>
  
  <p><a href="https://i.stack.imgur.com/IuyHO.png" rel="noreferrer"><img src="https://i.stack.imgur.com/IuyHO.png" alt="enter image description here"></a></p>