i write a bot to decode qr-codes from user’s message and send im data back. now i have an error

cv2.error: OpenCV(4.10.0) D:aopencv-pythonopencv-pythonopencvmodulesobjdetectsrcqrcode.cpp:32: error: (-215:Assertion failed) !img.empty() in function 'cv::checkQRInputImage'

code:

@decode_router.message(lambda message: True)
async def decode(message: Message):
    await main.bot.send_message(message.chat.id, "Decoding...")
    fl = main.bot.download_file(message.photo, f"qrcode{message.from_user.id}.png")
    fl = FSInputFile(f"qrcode{message.from_user.id}.png", f"qrcode{message.from_user.id}.png")
    img = cv2.imread(f"qrcode{message.from_user.id}.png")
    detector = cv2.QRCodeDetector()
    data, vebb, f  = detector.detectAndDecode(img)
    await message.reply(data)

what that error means and what should i do to fix it?