PIL image not displaying inside function

Question

I am trying to display photo.png in two different ways: first inside the mainloop and then inside a function called show_photo_in_another_window(). The code is identical in both. The image displays correctly inside mainloop but the function gives a blank window. Why? How to fix it? Thanks in advance. I am using Python 3.8.

import tkinter as tk
from PIL import Image, ImageTk

def show_photo_in_another_window():
    win2 = tk.Toplevel()
    win2.title('Second window')
    photo = ImageTk.PhotoImage(Image.open("photo.png"))
    tk.Label(win2, image=photo).pack()


root = tk.Tk()

win = tk.Toplevel()
win.title('First window')
photo = ImageTk.PhotoImage(Image.open("photo.png"))
tk.Label(win, image=photo).pack()

show_photo_in_another_window()

root.mainloop()