how should I use the function orb.detectAndCompute() correctly

edit

import cv2 as cv

imgpath='testImg.jpg'
img=cv.imread(imgpath,cv.IMREAD_GRAYSCALE)

orb=cv.ORB_create()
keypoints,descriptor=orb.detectAndCompute(img,None)

error: (-215) The data should normally be NULL! in function NumpyAllocator::allocate