title: 'invalid value encountered in double_scalars' warning, possibly numpy
body: <p>As I run my code I get these warnings, always in groups of four, sporadically. I have tried to locate the source by placing debug messages before and after certain statements to pin-point its origin.</p>
  
  <pre><code>Warning: invalid value encountered in double_scalars
  Warning: invalid value encountered in double_scalars
  Warning: invalid value encountered in double_scalars
  Warning: invalid value encountered in double_scalars
  </code></pre>
  
  <p>Is this is a Numpy warning, and what is a double scalar?</p>
  
  <p>From Numpy I use</p>
  
  <pre><code>min(), argmin(), mean() and random.randn()
  </code></pre>
  
  <p>I also use Matplotlib</p>