Assertion Failed Opencv Python unlock

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I am trying to run opencv-python==3.3.0.10 on a macOS 10.12.6 to read from a file and show the video in a window. I have exactly copied the code from here http://opencv-python-tutroals.readthe..., section 'Playing' video from file.

The code runs correctly and shows the video, however after termination of the video it breaks the program, giving the following error:

Assertion failed: (ec == 0), function unlock, file /BuildRoot/Library/Caches/com.apple.xbs/Sources/libcxx/libcxx-307.5/src/mutex.cpp, line 48.

Does anyone have any idea of what might have cause

Code snippet for your convenience

cap = cv2.VideoCapture('vtest.avi')

while(cap.isOpened()):
    ret, frame = cap.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()