When I add data to the Redux store, result is null

I really don't understand how to add data to my store, nothing complicated I don't understand, I looked at a lot of tutorials and resources but nothing works, the best I have is null in my store with the current userReducer.

My store :

My userReducer :

import {GET_USER, GET_USERS, ADD_USER} from "../actions/userAction";

export default function userReducer(state = {}, action) {
    switch (action.type) {
        case GET_USER:
            return action.payload

        case GET_USERS:
            return action.payload


        case ADD_USER:
            return [
                ...state,
                action.payload
            ]

        default :
            return state
    }
}

Thank you very much for your help

---EDIT---

        case GET_USERS:
            return {
                users: action.payload
            }
        case ADD_USER:
            return {
                users: [
                    ...state.users,
                    action.payload
                ]
            }