title: Error in numpy nanmean
body: <p>I have an array with floats and NaNs yet when I call <code>numpy.nanmean()</code> on the array I get <code>NaN</code>as result. How is this possible?</p>
  
  <p>This is a sample of the array <code>ages</code></p>
  
  <pre><code>[22.0 38.0 26.0 35.0 35.0 nan 54.0 2.0 27.0 14.0 4.0 58.0 20.0 39.0 14.0
   55.0 2.0 nan 31.0 nan 35.0 34.0 15.0 28.0 8.0 38.0 nan 19.0 nan nan 40.0
   nan nan 66.0 28.0 42.0 nan 21.0 18.0 14.0 40.0 27.0 nan 3.0 19.0 nan nan
   nan nan 18.0 7.0 21.0 49.0 29.0 65.0 nan 21.0 28.5 5.0 11.0 22.0 38.0 45.0
  ...
  </code></pre>
  
  <p>the shape</p>
  
  <pre><code>(891,)
  </code></pre>
  
  <p>and <code>np.nanmean(ages)</code> returns <code>nan</code></p>