title: Error testing reducer initial state
body: <p>I'm receiving an error when trying to do a basic reducer test such as those found on the redux docs: <a href="https://redux.js.org/recipes/writing-tests#reducers" rel="nofollow noreferrer">https://redux.js.org/recipes/writing-tests#reducers</a></p>
  
  <pre><code>TypeError: Cannot read property 'GROUP_LIST_REQUEST' of undefined
  
    22 | 
    23 |   switch (action.type) {
  &gt; 24 |     case GROUP_LIST_REQUEST:
    25 |       return {
    26 |         ...state,
    27 |         fetching: true,
  
    at group (app/reducers/group.js:24:10)
    at node_modules/redux/lib/combineReducers.js:53:24
    at Array.forEach (native)
    at assertReducerShape (node_modules/redux/lib/combineReducers.js:51:25)
    at combineReducers (node_modules/redux/lib/combineReducers.js:107:5)
    at Object.&lt;anonymous&gt; (app/reducers/all.js:23:16)
    at Object.&lt;anonymous&gt; (app/store.js:31:1)
    at Object.&lt;anonymous&gt; (app/models/api.js:31:1)
    at Object.&lt;anonymous&gt; (app/actions/group.js:1:1)
    at Object.&lt;anonymous&gt; (app/reducers/group.js:1:1)
    at Object.&lt;anonymous&gt; (__tests__/groups/reducer.js:6:1)
  </code></pre>
  
  <p>The stack trace seems to imply that the whole app is being run, when I just want to run my reducer method in isolation.</p>
  
  <p>But even weirder is that it can't find my action type in the reducer switch that I'm not even using in the test.</p>
  
  <pre><code>import reducer from '../../app/reducers/group';
  import * as actions from '../../app/actions/group';
  
  describe('group reducer', () =&gt; {
    test('should return the initial state', () =&gt; {
      expect(reducer(undefined, {})).toEqual({
        selected: null,
        fetching: false,
        fetched: false,
        error: null,
        items: [],
      });
    });
  });
  </code></pre>
  
  <p>Reducer:</p>
  
  <pre><code>import {
    GROUP_LIST_REQUEST,
    GROUP_LIST_SUCCESS,
    GROUP_LIST_FAILURE,
   } from '../actions/group';
  
  export const initialState = {
    selected: null,
    fetching: false,
    fetched: false,
    error: null,
    items: [],
  };
  
  export default function group(state = initialState, action) {
    console.log(action);
  
    switch (action.type) {
      case GROUP_LIST_REQUEST:
        return {
          ...state,
          fetching: true,
          fetched: false,
        };
      case GROUP_LIST_SUCCESS:
        return {
          ...state,
          fetching: false,
          fetched: true,
          items: action.groupList === undefined ? [] : action.groupList,
        };
      case GROUP_LIST_FAILURE:
        return {
          ...state,
          fetching: false,
          fetched: false,
        };
      default:
        return state;
    }
  }
  </code></pre>
  
  <p>When I log the action I get <code>{ type: '@@redux/INIT' }</code> which is weird because for the test I am passing an empty object.</p>
  
  <p>EDIT: I am using Jest to test, maybe it has to do with the configs?</p>