title: OpenCV NoneType object has no attribute shape
body: <p>Hello I'm working on Raspberry Pi with OpenCV. I want to try a tutorial which is ball tracking in link
  <a href="http://www.pyimagesearch.com/2015/09/14/ball-tracking-with-opencv/" rel="noreferrer">http://www.pyimagesearch.com/2015/09/14/ball-tracking-with-opencv/</a></p>
  
  <p>But when I compile it, i get an error: 'NoneType' object has no attribute 'shape'.</p>
  
  <p>What should I do?</p>