title: or-clause with date and null-values in python
body: <p>I have following df:</p>
  <pre><code>   date_from    date_to      birth_date    death_date
  0  2016-01-10   2019-06-05   2015-02-15    2018-07-25
  1  2016-05-11   2020-06-13   2014-03-07    2020-07-11
  2  2016-02-23   Nat          2014-03-07    2019-06-08
  3  2015-12-08   Nat          2014-03-07    2019-06-08
  </code></pre>
  <p>I'm trying to select all cases where date_to &gt; death_date OR where date_to = Nat.</p>
  <p>I've tried following code:</p>
  <pre><code>df = df[(df['date_to'] &gt; df['death_date']) | (df[df['DATE_TO'].isnull()])]
  </code></pre>
  <p>but I get following error-message</p>
  <pre><code>'TypeError: cannot compare a dtyped [float64] array with a scalar of type [bool]'
  </code></pre>
  <p>and I don't really know how to work around this problem.</p>