Hello, I am working on the image face recognition project. I am getting error as follows. Please guide.

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Error description- Traceback (most recent call last): File "C:\Users\om\Desktop\Program\prac.py", line 7, in <module> gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) #inputimage converted to grayscale cv2.error: OpenCV(4.4.0) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-95hbg2jt\opencv\modules\imgproct\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.emptColor'y() in function 'cv::cvtColor'