python +OpenCV 裁剪图片时报错

import cv2 as cv

template = r"D:\304550461.jpg"
tp = cv.imread(template)
a = tp[2:3, 6:8]

其中a = tp[2:3, 6:8]报错
TypeError: 'NoneType' object is not subscriptable
哪里出了问题？