title: pandas Remove rows with non-blank values per a field from a dataframe (Python 3.4/IPython)
body: <p>I am trying to create a new dataframe (ascombnoa) from an existing (ascomb) one where values of specific column (Discipline) are blank. I'm assuming that this is a string field as other values are words like "Math" and "Reading."</p>
  
  <p>I tried this:</p>
  
  <pre><code>ascombnoa = ascomb[ascomb.Discipline] == ""
  </code></pre>
  
  <p>and it returned this:
  ValueError: cannot index with vector containing NA / NaN values</p>
  
  <p>Any feedback would be greatly appreciated.</p>
  
  <p>Thanks!</p>