我正在尝试运行用于面部识别的代码并出现以下错误

Question

我正在尝试运行以下代码进行面部识别：

import cv2

face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')

def detect(gray, frame):
    faces = face_cascade.detectMultiScale(gray, 1.3, 5)
    for (x, y, w, h) in faces:
        cv2.rectangle(frame, (x, y), (x+w,y+h), (255, 0, 0), 2)
        roi_gray = gray[y:y+h, x:x+w]
        roi_color = frame[y:y+h, x:x+w]
        eyes = eye_cascade.detectMultiScale((roi_gray, 1.1, 3))
        for (ex, ey, ew, eh) in eyes:
            cv2.rectangle(roi_color, (ex, ey), (ex+ew,ey+eh), (0, 255, 0), 2)
    return frame

video_capture = cv2.VideoCapture(1)
while True:
    _,frame = video_capture.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    canvas = detect(gray, frame)
    cv2.imshow('Video', canvas)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
video_capture.release()
cv2.destroyAllWindows()

，并且出现以下错误： OpenCV（4.1.2）C：\ projects \ opencv-python \ opencv \ modules \ imgproc \ src \ color.cpp：182：错误：（-215：断言失败）函数'cv :: _ src.empty（） cvtColor'