Jest测试Redux Toolkit组件时报Store does not have a valid reducer错误

我已经排查这个问题很久了，但始终没找到错误根源。
我当前正在测试一个依赖Redux store获取用户相关状态的组件。
为了适配项目后续扩容的架构需求，我将独立reducers、combineReducer和Store拆分到了不同文件中维护。
目前我只写了一个简单测试，用来验证组件是否能正常渲染：

test('Component renders', () => {
    const test = shallow(<Provider store={mockStore}>
        <UserPage />
    </Provider>)
    expect(test).not.toBeNull()
})

我正在反向追溯测试通过但批量报错的原因：
将所有相关逻辑都写在测试文件内时，代码可以正常运行：

import { userObject } from "../../constants";
import { combineReducers, configureStore, createSlice } from '@reduxjs/toolkit';
import { Provider } from 'react-redux';
import UserPage from './UserPage';

const initialState = {
    user: userObject,
    loadingUser: false,
};
const userSlice = createSlice({
    name: 'user',
    initialState: initialState,
    reducers: {
        setUser(state, action) {
            state.user = action.payload
        },
        setLoadingUser(state) {
            state.loadingUser = !state.loadingUser
        }
    }
})

const reducers = combineReducers({
    user: userReducer
});

const mockStore = configureStore({ reducer: reducers })

test('Component renders', () => {
    const test = shallow(<Provider store={mockStore}>
        <UserPage />
    </Provider>)
    expect(test).not.toBeNull()
})

这种实现显然不合理，我需要测试实际业务的reducer，因此我做了如下调整来排查失败原因：

import userReducer from '../PATH/userReducer'
import { userObject } from "../../constants";

const reducers = combineReducers({
    user: userReducer
});
const mockStore = configureStore({ reducer: reducers })

test('Component renders', () => {
    const test = shallow(<Provider store={mockStore}>
        <UserPage />
    </Provider>)
    expect(test).not.toBeNull()
})

我编写的Reducer代码如下：

import { createSlice } from "@reduxjs/toolkit";

const initialState = {
    user: userObject,
    loadingUser: false,
};

const userSlice = createSlice({
    name: 'user',
    initialState: initialState,
    reducers: {
        setUser(state, action) {
            state.user = action.payload
        },
        setLoadingUser(state) {
            state.loadingUser = !state.loadingUser
        }
    }
})

export default userSlice.reducer;

调整后测试依然可以通过，但控制台抛出了大量错误，后续测试会因为reducer为undefined而失败。
控制台报错信息如下：

console.error
Store does not have a valid reducer. Make sure the argument passed to combineReducers is an object whose values are reducers.

  at warning (node_modules/redux/lib/redux.js:402:13)
  at currentReducer (node_modules/redux/lib/redux.js:525:9)
  at dispatch (node_modules/redux/lib/redux.js:296:22)
  at apply (node_modules/redux/lib/redux.js:382:3)
  at node_modules/redux/lib/redux.js:658:31
  at createStore (node_modules/redux/lib/redux.js:162:12)
  at configureStore (node_modules/@reduxjs/toolkit/dist/redux-toolkit.cjs.development.js:536:12)

注意：硬编码userObject无法解决该问题。

内容的提问来源于stack exchange，提问作者Jeppe Christensen