title: Error when using OpenCV face detection
body: <p>I am try to get face detection working on using a python script and OpenCV but I keep getting this error:</p>
  
  <pre><code>Traceback (most recent call last)
      File "face_detect.py", line 19, in &lt;module&gt;
          cascade = cv.Load(options, cascade)
  TypeError: OpenCV returned NULL
  </code></pre>
  
  <p>The code I used is here: <a href="https://github.com/mitchtech/py_servo_facetracker" rel="nofollow">https://github.com/mitchtech/py_servo_facetracker</a></p>
  
  <p>I don know what is causing this error or how to fix it. Thanks in advance. </p>