Adding babel-plugin-rewire

Question

I'm using https://github.com/mxstbr/react-boilerplate and want to mock and spy on my imports for unit tests. I have imported babel-plugin-rewire as dev dependency in my package.json

 "babel-plugin-rewire": "^1.0.0",

I have also added babel-plugin-rewire as a test plugin

"test": {
    "plugins": [
      "istanbul","babel-plugin-rewire"
    ]
  }

In my babel.test part in package.json.

And finally in my webpack.test.babel.js I have added

loader: 'babel?plugins=babel-plugin-rewire',

Then when I run

npm run test

I get a weird error saying

PhantomJS 2.1.1 (Mac OS X 0.0.0) ERROR
  TypeError: undefined is not an object (evaluating 'cov_1uiq851gv7.b[1][0]')
  at webpack:///app/reducers.js:36:4 <- test-bundler.js:51890

What have I missed? Has someone setup react-boilerplate with babel-plugin-rewire ?

The weird part is that reducers.js which gives there error ar excluded in my test.bundler.js Adding reducers.js

import { combineReducers } from 'redux-immutable';
import { fromJS } from 'immutable';
import { LOCATION_CHANGE } from 'react-router-redux';
import languageProviderReducer from 'containers/LanguageProvider/reducer';
import driverReducer from 'containers/DriverList/reducer';
import vehicleReducer from 'containers/VehicleList/reducer';

// Initial routing state
const routeInitialState = fromJS({
  locationBeforeTransitions: null,
});


function routeReducer(state = routeInitialState, action) {
  switch (action.type) {
    /* istanbul ignore next */
    case LOCATION_CHANGE:
      return state.merge({
        locationBeforeTransitions: action.payload,
      });
    default:
      return state;
  }
}

export default function createReducer(asyncReducers) {
  return combineReducers({
    route: routeReducer,
    driverList: driverReducer,
    vehicleList: vehicleReducer,
    language: languageProviderReducer,
    ...asyncReducers,
  });
}