MOG background subtraction: history parameter not working

Question

I'm trying to use MOG background subtraction but the "history" function doesn't seem to work.
OpenCV 2.4.13
Python (2.7.6)

Observation: The program appears to use the very first frame it captures for all future background subtractions.
Expectation: The background should slowly evolve based on the "history" parameter, so that if the camera angle changes, or if a person/object leaves the field-of-view, the "background" image will change accordingly.

import cv2
import numpy as np

cap = cv2.VideoCapture(0)
mog = cv2.BackgroundSubtractorMOG(history=10, nmixtures=5, backgroundRatio=0.25)

while True:
    ret, img1 = cap.read()

    if ret is True:
        cv2.imshow("original", img1)

        img_mog = mog.apply(img1)
        cv2.imshow("mog", img_mog)

    if cv2.waitKey(10) & 0xFF == ord(q):
        break

video_capture.release()
cv2.destroyAllWindows()

Thank you in advance for your help!