[Python] Why cv.convexityDefects fails in this example? Is it a bug?

This script:

import numpy as np, cv2 as cv

contour = np.array([[0, 0], [1, 0], [1, 1], [0.5, 0.2], [0, 0]], dtype='f4')
hull = cv.convexHull(contour, returnPoints=False)
defects = cv.convexityDefects(contour, hull)


fails and produces this error message:

File "/home/paul/upwork/pickleball/code/so-65-ocv-convexity-defects.py", line 5, in <module>
defects = cv.convexityDefects(contour, hull)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
cv2.error: OpenCV(4.11.0) /io/opencv/modules/imgproc/src/convhull.cpp:319: error: (-215:Assertion failed) npoints >= 0 in function 'convexityDefects'


What is the reason? Here is a plot of contour:



And hull is:

[[0]
[1]
[2]]

Continue reading...