opencv捕捉视频代码哪儿错了（在书上复制的代码 但报错了）

Question

-*- coding: utf-8 -*-

"""
Created on Fri Jan 3 21:06:22 2014

@author: duan
"""

import numpy as np
import cv2
cap = cv2.VideoCapture(0)

while(True):

ret, frame = cap.read()


gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)


cv2.imshow('frame',gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
    break

cap.release()
cv2.destroyAllWindows()

报错截图