title: Pandas: return NaN rows
body: <p>I am trying to return a df that contains all of the <code>NaN</code> values for <code>column == years_exp</code> so that I can identify the corresponding <code>id.thomas</code> (basically I'm debugging some data that I parsed by hand).  I also need to return a df with all <code>min</code> values. This is what I have tried so far:</p>
  
  <pre><code>rr.head(5)
  
      years   id.thomas   years_exp
  55  2005          2     17
  56  2006          2     18
  57  2007          2     19
  58  2008          2     20
  59  2009          2     21
  
  c = rr
  c = c[c.years_exp == 'NaN']
  </code></pre>
  
  <p>Error:</p>
  
  <blockquote>
    <p>TypeError: invalid type comparison</p>
  </blockquote>
  
  <p>I'm using syntax that I copied from a youtube video on Pandas. Does anyone have an idea about the error?</p>