我得到一个错误，TypeError：src不是numpy数组，也不是标量。

Traceback says 
Traceback (most recent call last):
  File "img.py", line 19, in <module>
    gray = cv2.cvtColor(images,cv2.COLOR_BGR2GRAY)
TypeError: src is not a numpy array, neither a scalar

我在img.py上写的

import numpy as np
import cv2
import glob
from PIL import Image

# termination criteria
criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)

# prepare object points, like (0,0,0), (1,0,0), (2,0,0) ....,(6,5,0)
objp = np.zeros((6*7,3), np.float32)
objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)

# Arrays to store object points and image points from all the images.
objpoints = [] # 3d point in real world space
imgpoints = [] # 2d points in image plane.

images = Image.open('photo.jpg')

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

# Find the chess board corners
ret, corners = cv2.findChessboardCorners(gray, (7,6),None)

# If found, add object points, image points (after refining them)
if ret == True:
   objpoints.append(objp)

   corners2 = cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
   imgpoints.append(corners2)

   # Draw and display the corners
   img = cv2.drawChessboardCorners(img, (7,6), corners2,ret)
   cv2.imshow('img',img)
   cv2.waitKey(500)

cv2.destroyAllWindows()

我搜索这个错误，我发现这个问题exceptions.TypeError: src is not a numpy array, neither a scalar，但我已经在cvtColor方法中使用了cv2.COLOR_BGR2GRAY，所以我真的不明白为什么会发生这个错误。我真的不明白src的含义。我的代码有什么问题？我该怎么解决这个问题？我引用了这个opencv文档https://docs.opencv.org/3.4.0/d9/df8/tutorial_root.html。