Выдает ошибку, что делать? Error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'

# install opencv "pip install opencv-python"
import cv2

# distance from camera to object(face) measured
# centimeter
Known_distance = 76.2

# width of face in the real world or Object Plane
# centimeter
Known_width = 14.3

# Colors
GREEN = (0, 255, 0)
RED = (0, 0, 255)
WHITE = (255, 255, 255)
BLACK = (0, 0, 0)

# defining the fonts
fonts = cv2.FONT_HERSHEY_COMPLEX

# face detector object
face_detector = cv2.CascadeClassifier("haarcascade_frontalface_default.xml")

# focal length finder function
def Focal_Length_Finder(measured_distance, real_width, width_in_rf_image):

# finding the focal length
focal_length = (width_in_rf_image * measured_distance) / real_width
return focal_length

# distance estimation function
def Distance_finder(Focal_Length, real_face_width, face_width_in_frame):

distance = (real_face_width * Focal_Length)/face_width_in_frame

# return the distance
return distance


def face_data(image):

face_width = 0 # making face width to zero

# converting color image to gray scale image
gray_image = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)

# detecting face in the image
faces = face_detector.detectMultiScale(gray_image, 1.3, 5)

# looping through the faces detect in the image
# getting coordinates x, y , width and height
for (x, y, h, w) in faces:

# draw the rectangle on the face
cv2.rectangle(image, (x, y), (x+w, y+h), GREEN, 2)

# getting face width in the pixels
face_width = w

# return the face width in pixel
return face_width


# reading reference_image from directory
ref_image = cv2.imread("Ref_image.png")

# find the face width(pixels) in the reference_image
ref_image_face_width = face_data(ref_image)

# get the focal by calling "Focal_Length_Finder"
# face width in reference(pixels),
# Known_distance(centimeters),
# known_width(centimeters)
Focal_length_found = Focal_Length_Finder(
Known_distance, Known_width, ref_image_face_width)

print(Focal_length_found)

# show the reference image
cv2.imshow("ref_image", ref_image)

# initialize the camera object so that we
# can get frame from it
cap = cv2.VideoCapture(0)

# looping through frame, incoming from
# camera/video
while True:

# reading the frame from camera
_, frame = cap.read ()

# calling face_data function to find
# the width of face(pixels) in the frame
face_width_in_frame = face_data(frame)

# check if the face is zero then not
# find the distance
if face_width_in_frame != 0:

# finding the distance by calling function
# Distance finder function need
# these arguments the Focal_Length,
# Known_width(centimeters),
# and Known_distance(centimeters)
Distance = Distance_finder(
Focal_length_found, Known_width, face_width_in_frame)

# draw line as background of text
cv2.line(frame, (30, 30), (230, 30), RED, 32)
cv2.line(frame, (30, 30), (230, 30), BLACK, 28)

# Drawing Text on the screen
cv2.putText(
frame, f"Distance: {round(Distance,2)} CM", (30, 35),
fonts, 0.6, GREEN, 2)

# show the frame on the screen
cv2.imshow("frame", frame)

# quit the program if you press 'q' on keyboard
if cv2.waitKey(1) == ord("q"):
break

# closing the camera
cap.release()

# closing the windows that are opened
cv2.destroyAllWindows()