I got a error "Incorrect type of self (must be 'Feature2D' or its derivative)" from line7(keypoints~~)

edit

import cv2 as cv import numpy as np

im = cv.imread("mini.jpg", cv.IMREAD_GRAYSCALE)

detector = cv.SimpleBlobDetector()
keypoints = detector.detect(im)
im_with_keypoints = cv.drawKeypoints(im, keypoints, np.array([]), (0,0,255), cv.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)

cv.imshow("keypoints", im_with_keypoints)
k = cv.waitKey(0)
cv.destroyAllWindows()