Draw Contours error
I am trying to draw contour for the objects in the image

```
(_, contours) = cv2.findContours(binary, cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

# draw contours over original image
length = len(contours)
for c in range(length):
    cv2.drawContours(img,contours,c,(255,0,0),3)

cv2.namedWindow("output", cv2.WINDOW_NORMAL)
cv2.imshow("output", img)
cv2.waitKey(0)

```

I expect the all the contours to be drawn but the actual result that i get is 
error: 

> 
> (-215:Assertion failed) npoints > 0 in function 'drawContours'
> 
> 
>