[Python] How do I get icon of window as PNG using python xlib?

I am working on a window manager made in Python 3 using python-xlib. When I tried to implement displaying the icons (as a PNG) on a window as a 16x16 image, it worked with this dummy image, but when I tried to actually get the window's icon, it returned None.

from Xlib.display import Display
from Xlib import X
from PIL import Image

def getIcon(window, display):
icon_data = window.get_full_property(
display.intern_atom('_NET_WM_ICON'),
X.AnyPropertyType
).value

width = icon_data[0]
height = icon_data[1]

image = Image.frombytes(
'RGBA', (width, height), bytes(icon_data[2:]), 'raw', 'BGRA'
)
return image

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