title: numpy pad array with nan, getting strange float instead
body: <p>I'm trying to pad an array with <code>np.nan</code></p>
  
  <pre><code>import numpy as np
  print np.version.version
  # 1.10.2
  combine = lambda real, theo: np.vstack((theo, np.pad(real, (0, theo.shape[0] - real.shape[0]), 'constant', constant_values=np.nan)))
  real = np.arange(20)
  theoretical = np.linspace(0, 20, 100)
  result = combine(real, theoretical)
  np.any(np.isnan(result))
  # False
  </code></pre>
  
  <p>Inspecting <code>result</code>, it seems instead of <code>np.nan</code>, the array is getting padded with <code>-9.22337204e+18</code>. What's going on here? How can I get <code>np.nan</code>?</p>