Why this happened?: SystemError: <class 'cv2.CascadeClassifier'> returned a result with an error set

[Thomas-stc]

Dear sir.
When I practice face detection by using the code as follows and an error happened as mentioned in Title.
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
face_cascade = cv2.CascadeClassifier('opencv-files/lbpcascade_frontalface.xml')
faces = face_cascade.detectMultiScale(gray, scaleFactor=1.0, minNeighbors=5)
I am sure that the the file "lbpcascade_frontalface.xml" has in the folder "opencv-files" which I created.
by the way, I got "lbpcascade_frontalface.xml" from my colleague because I don't know where to download this file on web.
I am looking forward for your answer, thank you very much.