Create store failed when use handleActions with empty reducerMap

I have a `todosReducer`:
```
import { handleActions } from 'redux-actions';

const initState = {}

export default handleActions({

}, initState)
```

Error message:
```
Uncaught Error: Reducer "todos" returned undefined during initialization. If the state passed to the reducer is undefined, you must explicitly return the initial state. The initial state may not be undefined.
```