OpenCV图像降噪给出：错误：-215：断言失败

使用下面的代码尝试去噪一个非常简单的图像。当打印出数据数组时，我得到以下结构，这是预期的，因为图像是灰度的：

[[ 62  62  63 ...  29  16   6]
 [ 75  90 103 ...  21  16  12]
 [ 77 100 118 ...  29  29  30]
 ...
 [ 84  68  56 ...  47  50  53]
 [101  94  89 ...  40  44  48]

这是代码和相关的错误，这时我有点卡住了。有什么建议？

import cv2
from matplotlib import pyplot as plt

img = cv2.imread(path,0)
dst = cv2.fastNlMeansDenoising(img,None,10,10,7,21)

plt.subplot(211),plt.imshow(dst)
plt.subplot(212),plt.imshow(img)
plt.show()

____________________________________________________________________
runfile(___, wdir='G:/James Alexander/Python Programs')
Traceback (most recent call last):

  File "<ipython-input-127-ce832752c183>", line 1, in <module>
    runfile('G:/James Alexander/Python Programs/Noiseremoval.py', wdir=___)

  File "___", line 704, in runfile
    execfile(filename, namespace)

  File "___", line 108, in execfile
    exec(compile(f.read(), filename, 'exec'), namespace)

  File "___", line 13, in <module>
    dst = cv2.fastNlMeansDenoising(img,None,10,10,7,21)

error: OpenCV(4.1.0) C:\projects\opencv-python\opencv\modules\photo\src\denoising.cpp:120: error: (-215:Assertion failed) hn == 1 || hn == cn in function 'cv::fastNlMeansDenoising'