错误 AttributeError: 'NoneType' object has no attribute 'astype'

# 自定义函数数高斯模糊
import cv2 as cv
import numpy as np
from matplotlib import pyplot as plt


path = 'main.jpg'
img = cv.imread(path, 0)

# 自定义 根据窗口大小（10， 10）来计算高斯函数标准差
kernel_size = (7, 7)
sigma = 2
blur = cv.GaussianBlur(img, kernel_size, sigma)

img_new = blur.astype(np.uint8)

plt.subplot(121), plt.imshow(img), plt.title('main')
plt.xticks(), plt.yticks()
plt.subplot(122), plt.imshow(blur), plt.title('median')
plt.xticks(), plt.yticks()
plt.show()

# cv.imshow('img', blur)
# cv.waitKey(0)

错误原因：path的路径中没有对应的图片，所以类型当然为空了~~~