title: Error using knnMatch with OpenCV+Python
body: <p>I want to match two pictures using Python+OpenCV. I have used SURF to extract keypoints and descriptors from both of them. Now, I need to match these descriptors and for this reason I decided to use Flann Matcher. </p>
  
  <pre><code>flann_params = dict(algorithm = FLANN_INDEX_KDTREE,trees = 4)    
  matcher = cv2.FlannBasedMatcher(flann_params, {})
  </code></pre>
  
  <p>But when I try to use knnMatch with descriptors (desc1, desc2), openCV throws an exception.</p>
  
  <pre><code>raw_matches=matcher.knnMatch(np.asarray(desc1),np.asarray(desc2), 2)
  </code></pre>
  
  <p>The exception is the following:</p>
  
  <pre><code>raw_matches=matcher.knnMatch(np.asarray(desc1),np.asarray(desc2), 2) #2
  cv2.error: /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_graphics_opencv/opencv/work/OpenCV-2.4.2/modules/flann/src/miniflann.cpp:299: error: (-210) type=6
   in function buildIndex_
  </code></pre>
  
  <p>How I could use knnMatch correctly? Is it a Bug?</p>