https://github.com/opencv/opencv/blob/master/samples/python/tutorial_code/video/optical_flow/optical_flow.py

Traceback (most recent call last):
File "optical_flow.py", line 37, in
frame_gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.5.3) /private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/pip-req-build-ucsrqn3m/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

https://github.com/opencv/opencv/blob/master/samples/python/tutorial_code/video/optical_flow/optical_flow_dense.py

This problem occurs when the video is read to the last frame

Traceback (most recent call last):
File "optical_flow_dense.py", line 10, in
next = cv.cvtColor(frame2,cv.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.5.3) /private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/pip-req-build-ucsrqn3m/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

System information (version)

• OpenCV => ❔
• Operating System / Platform => ❔
• Compiler => ❔

Detailed description

Steps to reproduce

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc