title: NaN values when adding two columns
body: <p>I have two dataframes with different indexing that I want to sum the same column from the two dataframes. 
  I tried the following but gives NaN values</p>
  
  <pre class="lang-py prettyprint-override"><code>result['Anomaly'] = df['Anomaly'] + tmp['Anomaly']
  </code></pre>
  
  <pre><code>df
      date           Anomaly
  0 2018-12-06         0
  1 2019-01-07         0
  2 2019-02-06         1
  3 2019-03-06         0
  4 2019-04-06         0
  
  tmp
      date           Anomaly
  0 2018-12-06         0
  1 2019-01-07         1
  4 2019-04-06         0
  
  result
      date           Anomaly
  0 2018-12-06        0.0
  1 2019-01-07        NaN
  2 2019-02-06        1.0
  3 2019-03-06        0.0
  4 2019-04-06        0.0
  </code></pre>
  
  <p>What I want is actually:</p>
  
  <pre><code>result
      date           Anomaly
  0 2018-12-06         0
  1 2019-01-07         1
  2 2019-02-06         1
  3 2019-03-06         0
  4 2019-04-06         0
  </code></pre>