A case reducer on a non-draftable value must not return undefined

I'm working on my project @reduxjs/toolkit and created UserSlice like below.

import { createSlice, PayloadAction } from "@reduxjs/toolkit";
import { User } from "../../models";

export type UserState = User | null;

export const initialUserState: UserState = null;

const UserSlice = createSlice({
  name: "User",
  initialState: initialUserState,
  reducers: {
    receiveUser: (state: UserState, action: PayloadAction<UserState>) => {
      state = action.payload;
    },
    clearUser: (state: UserState, _action: PayloadAction<void>) => {
      state = null;
    },
    signIn: (
      _,
      _action: PayloadAction<{
        email: string;
        password: string;
      }>
    ) => {},
    signInWithToken: (_, _action: PayloadAction<void>) => {},
  },
});

export default UserSlice;

signIn signInWithToken are action creators just for taking actions in UserSaga module, so they do nothing.

When I dispatch signIn action, below error occurs.

index.js:1 Error: A case reducer on a non-draftable value must not return undefined
    at createReducer.ts:163
    at Array.reduce (<anonymous>)
    at createReducer.ts:143
...

This code snippets pattern works fine my other projects, but this project. I can't figure out the reason of this error, so I'm asking this question. Are there any faults in my code?

I believe you're not returning the state in all of your reducers, because when using arrow functions objects need additional round brackets:

  reducers: {
    receiveUser: (state: UserState, action: PayloadAction<UserState>) => ({
      state: action.payload;
    }),
    clearUser: (state: UserState, _action: PayloadAction<void>) => ({
      state : null;
    }),
    signIn: (
      _,
      _action: PayloadAction<{
        email: string;
        password: string;
      }>
    ) => ({}),
    signInWithToken: (_, _action: PayloadAction<void>) => ({}),
  },