How can you replace entire state in Redux Toolkit reducer?

EDIT: The solution is to return state after I replace it completely (return state = {...action.payload})! But why? I don't need to return it when I replace the fields individually.

I'm working with the Redux Toolkit, which simplifies some Redux boilerplate. One thing they do is use Immer to allow you to directly 'modify' state (in fact, you're not). It works fine, except I don't know how to replace my section of state entirely. For instance, I want to do something like this

const reducer = createReducer({ s: '', blip: [] }, {

    [postsBogus.type]: (state, action) => {
        state = { ...action.payload };
    }

but state remains untouched. Instead I have to do this

[postsBogus.type]: (state, action) => {
    state.s = action.payload.s;
    state.blip = action.payload.blip;
}

Is there a way I can replace state entirely?