how to pass keypoints for orb.computeAsync

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Im trying to compute descriptors for the keypoints found, but Im getting segmentation fault error. Part of the problem is that there is very little documentation for opencv cuda in python. I am getting an image data, and using cv2.cuda.createGoodFeaturesToTrack() for getting keypoints. I want to calculate descriptors for the keypoints, and hence using computeAsync(). Here is my code -

def extract(img):
    #orb = cv2.cuda_ORB.create()
    orb = cv2.ORB_create()
    #converting to grey
    cuC1 = cv2.cuda_GpuMat()
    cuC1.upload(img)
    cuC1 = cv2.cuda.cvtColor(cuC1, cv2.COLOR_RGB2GRAY)
    #detection
    pts = cv2.cuda.createGoodFeaturesToTrackDetector(cv2.CV_8UC1)
    _kps = pts.detect(cuC1)
    npKps = _kps.download()
    npKps = np.int0(npKps)
    for n in npKps:
        #extraction
        kps = [cv2.KeyPoint(x=f.ravel()[0], y=f.ravel()[1], _size=20) for f in n]  #cpu keypoints, works well with non cuda orb
    kps, des = orb.computeAsync(cuC1, _kps)
    #return points and descriptors