无法从mapstatetoprops方法获取状态

// reducers.js
const initialState = {
  myReducer: {
    someValue: 'initial value'
  }
};

const rootReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'UPDATE_VALUE':
      return {
        ...state,
        myReducer: {
          ...state.myReducer,
          someValue: action.payload
        }
      };
    default:
      return state;
  }
};

export default rootReducer;

// actions.js
export const updateValue = (value) => ({
  type: 'UPDATE_VALUE',
  payload: value
});

// MyComponent.js
import React from 'react';
import { connect } from 'react-redux';
import { updateValue } from './actions';

const MyComponent = (props) => {
  return (
    <div>
      <p>{props.myState.someValue}</p>
      <button onClick={() => props.updateValue('new value')}>Update Value</button>
    </div>
  );
};

const mapStateToProps = (state) => {
  return {
    myState: state.myReducer
  };
};

const mapDispatchToProps = {
  updateValue
};

export default connect(mapStateToProps, mapDispatchToProps)(MyComponent);

// index.js
import React from 'react';
import ReactDOM from 'react-dom';
import { createStore } from 'redux';
import { Provider } from 'react-redux';
import rootReducer from './reducers';
import MyComponent from './MyComponent';

const store = createStore(rootReducer);

ReactDOM.render(
  <Provider store={store}>
    <MyComponent />
  </Provider>,
  document.getElementById('root')
);