错误：运行此程序时出现OpenCV（3.4.2）错误

Question

import cv2
import numpy as np

img = cv2.imread('opencv_logo.png',0)

img = cv2.medianBlur(img,5)

cimg = cv2.cvtColor(img,cv2.COLOR_GRAY2BGR)

circles = cv2.HoughCircles(img,cv2.HOUGH_GRADIENT,1,20,param1=50,param2=30,minRadius=0,maxRadius=0)

circles = np.uint16(np.around(circles))

for i in circles[0,:]:

  # draw the outer circle
  cv2.circle(cimg,(i[0],i[1]),i[2],(0,255,0),2)
  # draw the center of the circle
  cv2.circle(cimg,(i[0],i[1]),2,(0,0,255),3)


cv2.imshow('detected circles',cimg)

cv2.waitKey(0)

cv2.destroyAllWindows()

以下错误是什么？

error: OpenCV(3.4.2) D:\Build\OpenCV\opencv-3.4.2\modules\imgproc\src\hough.cpp:1737: error: (-215:Assertion failed) !_image.empty() && _image.type() == (((0) & ((1 << 3) - 1)) + (((1)-1) << 3)) && (_image.isMat() || _image.isUMat()) in function 'cv::HoughCircles'