python TypeError: 'NoneType' object has no attribute '__getitem__'

This time I am trying another example from Solem's blog. It's a module that detects lines and circles in an image by using the Hough transform. Here is the code (houghlines.py): ``` import numpy as np import cv2 """ Script using OpenCV's Hough transforms for reading images of simple dials. """ # load grayscale image im = cv2.imread("house2.jpg") gray_im = cv2.cvtColor(im, cv2.COLOR_RGB2GRAY) # create version to draw on and blurred version draw_im = cv2.cvtColor(gray_im, cv2.COLOR_GRAY2BGR) blur = cv2.GaussianBlur(gray_im, (0,0), 5) m,n = gray_im.shape # Hough transform for circles circles = cv2.HoughCircles(gray_im, cv2.cv.CV_HOUGH_GRADIENT, 2, 10, np.array([]), 20, 60, m/10)[0] # Hough transform for lines (regular and probabilistic) edges = cv2.Canny(blur, 20, 60) lines = cv2.HoughLines(edges, 2, np.pi/90, 40)[0] plines = cv2.HoughLinesP(edges, 1, np.pi/180, 20, np.array([]), 10)[0] # draw for c in circles[:3]: # green for circles (only draw the 3 strongest) cv2.circle(draw_im, (c[0],c[1]), c[2], (0,255,0), 2) for (rho, theta) in lines[:5]: # blue for infinite lines (only draw the 5 strongest) x0 = np.cos(theta)*rho y0 = np.sin(theta)*rho pt1 = ( int(x0 + (m+n)*(-np.sin(theta))), int(y0 + (m+n)*np.cos(theta)) ) pt2 = ( int(x0 - (m+n)*(-np.sin(theta))), int(y0 - (m+n)*np.cos(theta)) ) cv2.line(draw_im, pt1, pt2, (255,0,0), 2) for l in plines: # red for line segments cv2.line(draw_im, (l[0],l[1]), (l[2],l[3]), (0,0,255), 2) cv2.imshow("circles",draw_im) cv2.waitKey() # save the resulting image cv2.imwrite("res.jpg",draw_im) ``` When I execute the file in python by: ``` execfile('houghlines.py') ``` the following error comes up: ``` File "houghlines.py", line 24, in <module> lines = cv2.HoughLines(edges, 2, np.pi/90, 40)[0] TypeError: 'NoneType' object has no attribute '__getitem__' ``` Do you guys have any idea how to solve it? Thanks in advance.