opencv error

import cv2

cap = cv2.VideoCapture(0)
cap.set(3, 640) #WIDTH
cap.set(4, 480) #HEIGHT

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

while(True):
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    faces = face_cascade.detectMultiScale(gray, 1.3, 5)
    print(len(faces))
    # Display the resulting frame
    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)
         for (ex,ey,ew,eh) in eyes:
             cv2.rectangle(roi_color,(ex,ey),(ex+ew,ey+eh),(0,255,0),2)

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

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

Traceback (most recent call last):
File "C:/Users/Furkan/PycharmProjects/untitled3/app.py", line 16, in <module>
faces = face_cascade.detectMultiScale(gray, 1.3, 5)
cv2.error: OpenCV(4.1.1) C:\projects\opencv-python\opencv\modules\objdetect\src\cascadedetect.cpp:1689: error: (-215:Assertion failed) !empty() in function 'cv::CascadeClassifier::detectMultiScale'

how can ı to fix error