React Native TypeScript: accessing literals object return undefined is not an object (evaluating 'userPlanning[day]') when reloading

I'm very new in React Native. I'm stuck since 2 days. I tried to look for solution.I'm trying to fetch a plannings array from a database which as the following shape:

 export interface Plannings {    [day: string]: { session: string; when: string; time: string }[];}

When I fetch data for the first time I get the data but when I reload I get undefined is not an object (evaluating 'userPlanning[day]')

    const fetchPlannings = useCallback(async () => {        const planningsData = await AsyncStorage.getItem('plannings');        try {            if (planningsData) {                const planningStored = await JSON.parse(planningsData);                setPlannings(planningStored);                const nextSessionData = getNextSession(planningStored, user, frenchDay());                setNextSession({                    session: nextSessionData.session,                    time: nextSessionData.time,                    when: nextSessionData.when                })            }        } catch (e) {            console.log(e)        }    }, []);    useEffect(() => {        fetchPlannings();    }, []);

And here is the getNextSession method:

    const getNextSession = (plannings: Plannings[], user: User, day: string): Session => {        const userPlanning = plannings[user.group - 1];        const hours = new Date().getHours();        console.log(day);        if (hours >= 4 && hours <= 11) {            return userPlanning[day][0];        } else if (hours >= 12 && hours <= 20) {            return userPlanning[day][1];        } else {            return userPlanning[day][0];        }    }

Thanks for helping me =)