python beginner mouseclicking problem

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Hello, i'm learning the basics of opencv. I am working with the bit of code below. It generates a black image, i cannot click in some areas of the image without getting an error. Can anyone say why? seems to work ok on images that have the same value for x & y.

import cv2
import numpy as np


def click_event(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        blue = img[x, y, 0]
        green = img[x, y, 1]
        red = img[x, y, 2]
        print(blue, ' ', green, ' ', red)
        print(x, ' ', y)


img = np.zeros((577, 433, 3), np.uint8)

print(img.shape)
cv2.imshow('image', img)
cv2.setMouseCallback('image', click_event)

cv2.waitKey(0)
cv2.destroyAllWindows()