Python, OpenCv Array Error

I am trying to draw contours where I see motion in an image using python and cv2. I followed the tutorial here: http://www.steinm.com/blog/motion-detection-webcam-python-opencv-differential-images/ to get the motion part working, but I keep running into different array errors when I try different approaches to getting the contours drawn.

Here is my approach. I find the differential image and then find the contours on that image. Then I draw the contours on the differential image and then show it.

Here is the code:

import cv2
import numpy as np

#Find the differential image
def diffImg(t0, t1,t2):
    d1 = cv2.absdiff(t2, t1)
    d2 = cv2.absdiff(t1, t0)
    d_final = cv2.bitwise_and(d1,d2)
    d_binary = cv2.threshold(d_final, 35, 255,cv2.THRESH_BINARY)[1]
    d_blur =  cv2.blur(d_binary, (15,15))
    return d_blur

#Capture Video from camera
cam = cv2.VideoCapture(0)
s, img = cam.read()

window_name = "Movement Visual"
cv2.namedWindow(window_name, cv2.CV_WINDOW_AUTOSIZE)

#Read the first three images to find the differential image
t_minus = cv2.cvtColor(cam.read()[1], cv2.COLOR_RGB2GRAY)
t = cv2.cvtColor(cam.read()[1], cv2.COLOR_RGB2GRAY)
t_plus = cv2.cvtColor(cam.read()[1], cv2.COLOR_RGB2GRAY)

while s:
    #draw contours
    contours, hierarchy = cv2.findContours(diffImg(t_minus,t,t_plus),cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
    final_contour = cv2.drawContours(img,contours,-1,(250,250,250),2)
    cv2.imshow(window_name,final_contour)
    t_minus = t
    t = t_plus
    t_plus = cv2.cvtColor(cam.read()[1],cv2.COLOR_RGB2GRAY)

    key = cv2.waitKey(10)
    if key == 27: 
        cv2.destroyWindow(window_name)
        break
print "Bye"

#cv2.imwrite("image.png", diffImg(t_minus,t,t_plus)

Here is the error I am getting:

 line 28, in <module>
    cv2.imshow(window_name,final_contour)
error: C:\slave\WinInstallerMegaPack\src\opencv\modules\core\src\array.cpp:2482: error: (-206) Unrecognized or unsupported array type

Line 28 is where final_contour is declared in the while loop. I don't understand why I am getting this since it seems like I am essentially just swapping images between functions.

Thanks for any advice.