tkinter messagebox which turns your computer off

I'm trying to shut down the computer when the yes button of messagebox is clicked, but my code doesn't work somehow, and I don't know where is the problem

import tkinter as tk
from tkinter import messagebox
import os

res = messagebox.askquestion("Prezentácia bez názvu.pptx", "Failed to load. Do you wish to continue? ")

def shutdown():
    return os.system("shutdown /s /t 1")

def askMe():
    if res == 'yes':
        command=shutdown
    if res == 'no':
        command=shutdown