Opencv-python version4.5.x get(cv2.CAP_PROP_POS_MSEC)'s problem

Python
1

my code like below:


cam = cv2.VideoCapture(mp4_file)
while True:
    ret, frame = cam.read()
    if ret:
         ms = cam.get(cv2.CAP_PROP_POS_MSEC)

I want to get every frame’s time but the last six frames’ time ms was always 0
I tried to change some different mp4 files all had the same problem