redux-persist的getItem和setItem报错？

Question

redux-persist报错：Cannot read property 'getItem' of undefined 和 Cannot read property 'setItem' of undefined

按照教程的写法

const persistConfig = {

key: 'root',
storageSession,
stateReconciler: autoMergeLevel2

};

function configureStore () {

const persistedReducer = persistCombineReducers(persistConfig, rootReducer);
const store = createStore(persistedReducer, applyMiddleware(thunk));
const persistor = persistStore(store);
return {store, persistor};

}
module.exports = configureStore;

引入PersistGate组件
ReactDOM.render

(
    <Provider store={store}>
        <PersistGate loading={null} persistor={persistor}>
            <Router history={history}>
                {routes}
            </Router>
        </PersistGate>
    </Provider>
    , document.getElementById('app_root')
);

官网给的答案是：定制任何符合标准的存储api，实现以下方法：setItem getItem removeItem。（注意：这些方法必须支持承诺）
但是不知道在哪添加这些api，哪位大神能帮忙解答一下？