我一直在尝试用下面的opencv代码校准我的相机。我一直得到错误，error: (-215:Assertion failed) nimages > 0 in function 'cv::calibrateCameraRO'。我已经检查了图像是否正在使用cv.imshow()函数读取，并且我能够在代码循环时查看图像。我确实认为图像正在被馈送，地址是正确的。有什么线索能说明问题吗？

import numpy as np
import cv2 as cv
import glob
import pickle

chessboardSize = (9,6)
frameSize = (1500,2000)

# termination criteria
criteria = (cv.TERM_CRITERIA_EPS + cv.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((chessboardSize[0] * chessboardSize[1], 3), np.float32)
objp[:,:2] = np.mgrid[0:chessboardSize[0],0:chessboardSize[1]].T.reshape(-1,2)

size_of_chessboard_squares_mm = 20
objp = objp * size_of_chessboard_squares_mm

# 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 = glob.glob('\images\*.png')
# images = ['images\img0.png', 'images\img1.png', 'images\img10.png', 'images\img2.png', 'images\img3.png', 'images\img4.png', 'images\img5.png', 'images\img6.png', 'images\img7.png', 'images\img8.png', 'images\img9.png']
# images = ['images\img0.png', 'images\img1.png']

for image in images:

    img = cv.imread(image)
    gray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)

    # Find the chess board corners
    ret, corners = cv.findChessboardCorners(gray, chessboardSize, None)
    # print(ret)
    # print(corners)

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

        objpoints.append(objp)
        corners2 = cv.cornerSubPix(gray, corners, (11,11), (-1,-1), criteria)
        imgpoints.append(corners)

        # Draw and display the corners
        cv.drawChessboardCorners(img, chessboardSize, corners2, ret)
        cv.imshow('img', img)
        cv.waitKey(1000)

cv.destroyAllWindows()

############## CALIBRATION #######################################################

ret, cameraMatrix, dist, rvecs, tvecs = cv.calibrateCamera(objpoints, imgpoints, frameSize, None, None)