React redux: Cannot set on an immutable record

I am trying to update my redux model from my reducer. The model is an extension of an Immutable Record class. I am trying to update the Record with the set method:

import { List, Record } from 'immutable';
import { IFaqItem } from './api.models';
export interface IFaqsState {

loading?: boolean;

items?: List<IFaqItem>;

}

const faqsState = Record({

loading: false,

items: List()

});
class FaqsState extends faqsState implements IFaqsState {

loading: boolean;

items: List<IFaqItem>;
with(props: IFaqsState) {

this.set(‘loading’, props.loading);

return this;

}

}
export default FaqsState;

The reducer contains the following:

case ActionTypes.FAQS_GET_AJAX_RECEIVE:

let response: IFaqsGetResponse = action.payload.response && action.payload.response.response;

return state.with({

loading: false,

items: List(response)

});

However this gives me the following error:

Error: Cannot set on an immutable record.

UPDATE

When I change the reducer from:

let initial = new FaqsState();

const faqsReducer: Reducer<FaqsState> = (state = initial, action: AppActions) => {

to:

const faqsReducer: Reducer<FaqsState> = (state = null, action: AppActions) => {

state = new FaqsState();

it seems to work. Why is it not working when I give the initial state in as an argument?

#reactjs #redux