Property 'app' does not exist on type 'WritableDraft<{ channelId: null; channelName: null; }>'

I am trying to build a chat app with Tauri, React and Redux. I however am stuck on this error.

Property 'app' does not exist on type 'WritableDraft<{ channelId: null; channelName: null; }>'.

This is the code that produces this error:

import { PayloadAction, createSlice } from '@reduxjs/toolkit';
import type { RootState } from '../app/store';

export const appSlice = createSlice({
    name: 'app',
    initialState: {
        channelId: null,
        channelName: null,
    },
    reducers: {
        setChannelId: (state, action) => {
            state.app += action.payload;
        },
    },
});

export const { setChannelId } = appSlice.actions;

export const selectChannelId = (state: RootState) => state.app.channelId;
export const selectChannelName = (state: RootState) => state.app.channelName;

export default appSlice.reducer;

I tried adding app: null to the initialState like so:

    initialState: {
        app: null,
        channelId: null,
        channelName: null,

    },

This solved the error, but my app wouldn't render.

In case it's needed, here is my App.tsx:

import { useSelector } from "react-redux";
import { selectUser } from "./features/userSlice";

import "./App.css";
import Sidebar from "./Sidebar";
import Chat from "./Chat";

function App() {
  const user: any = useSelector(selectUser);

  return (
    <div className="app">
      {user ? (
        <>
        <Sidebar />
        <Chat />
        </>
      ) : (
        <h2>You need to log in</h2>
      )}

    </div>
  )
}

export default App;

And my store.ts:

import { configureStore } from '@reduxjs/toolkit';
import userReducer from '../features/userSlice';
import appReducer from '../features/appSlice';

export const store =  configureStore({
    reducer: {
        user: userReducer,
        app: appReducer,
    }
});

// Infer the `RootState` and `AppDispatch` types from the store itself
export type RootState = ReturnType<typeof store.getState>;

// Inferred type: {user: UserState}
export type AppDispatch = typeof store.dispatch;

I am stuck on this for a long time now, so all help would be great!

The shape of the "app" state slice is { channelId: null; channelName: null; }.

So the setChannelId reducer logic should be:

setChannelId: (state, action) => {
  state.channelId += action.payload;
},