isContourConvex always returns "False"?

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I have tried to use cv2.isContourConvex on several different shapes, but I get "False" all the time. I think I might have misunderstood how cv2.isContourConvex is working. Can anyone help?

Here is how I have tried to use it on the image below (which should be convex, right?):

img = cv2.imread('convex.png')

img_gray=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(img_gray,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)
image,contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE)
cnt = contours[0]

print cv2.isContourConvex(cnt)