OpenCV on MacOS, createTrackbar/getTrackbarPos causing segmentation fault

I'm new to Opencv and tried to run a trackbar on mac and it's giving me a segmentation fault. The code is:

import numpy as np
import cv2 as cv

def nothing(x):
    pass

img = np.zeros((300,512,3), np.uint8)

cv.namedWindow('image')
cv.createTrackbar('R', 'image', 0, 255, nothing)
cv.createTrackbar('G', 'image', 0, 255, nothing)
cv.createTrackbar('B', 'image', 0, 255, nothing)
switch = '0 : OFF \n1 : ON'
cv.createTrackbar(switch, 'image', 0, 1, nothing)

while True:
    cv.imshow('image', img)

    k = cv.waitKey(1)
    if k == 27:
        break
    r = cv.getTrackbarPos('R', 'image')
    g = cv.getTrackbarPos('G', 'image')
    b = cv.getTrackbarPos('B', 'image')
    s = cv.getTrackbarPos(switch, 'image')
    if s == 0:
        img[:] = 0
    else:
        img[:] = [b,g,r]

cv.destroyAllWindows()