[Python] How to calculate histogram of an image?

How can I solve the below error?

import cv2

img= cv2.imread('/home/monojit/Desktop/crop.jpg')
hsv= cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

hist= cv2.calcHist([hsv],[0,1],None,[256],[0,256])
print(hist)

cv2.imshow('img',hsv)
cv2.waitKey(0)
cv2.destroyAllWindows()


But on execution the get the following error:

Traceback (most recent call last):
File "/home/monojit/Desktop/hisCalc.py", line 6, in <module>
hist= cv2.calcHist([hsv],[0,1],None,[256],[0,256])
error: OpenCV(3.4.1) /home/monojit/Desktop/OpenCV/modules/imgproc/src/histogram.cpp:1782: error: (-215) csz == 0 || csz == dims in function calcHist


How do I solve the error?

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