title: Calculation between pandas dataframe returns NaN
body: <p>I have a pandas dataframe called <code>df_mod</code>. One variable of interest in this dataframe is called <code>Evap_mod</code>. When I use the command <code>print(df_mod['Evap_mod'])</code>, it returns:</p>
  
  <pre><code>2003-12-20 00:30:00    1.930664
  2003-12-21 00:30:00    1.789290
  2003-12-22 00:30:00    2.318347
  2003-12-23 00:30:00    1.741943
  2003-12-24 00:30:00    1.686124
  2003-12-25 00:30:00    1.852876
  2003-12-26 00:30:00    1.759650
  2003-12-27 00:30:00    1.566521
  2003-12-28 00:30:00    1.496039
  2003-12-29 00:30:00    1.540751
  2003-12-30 00:30:00    2.006475
  2003-12-31 00:30:00    1.920912
  Name: Evap_mod, Length: 729, dtype: float32
  </code></pre>
  
  <p>I have another pandas dataframe called <code>dff</code>. One variable of interest in this dataframe is called <code>PET_PT</code>. When I use the command <code>print(dff['PET_PT'])</code>, it returns:</p>
  
  <pre><code>2003-12-20    4.810697
  2003-12-21    4.739378
  2003-12-22    4.994467
  2003-12-23    5.138086
  2003-12-24    5.024226
  2003-12-25    4.937206
  2003-12-26    4.551416
  2003-12-27         NaN
  2003-12-28         NaN
  2003-12-29         NaN
  2003-12-30         NaN
  2003-12-31         NaN
  Freq: D, Name: PET_PT, Length: 729, dtype: float64
  </code></pre>
  
  <p>I would like to run the simple following calculation between those 2 variables:</p>
  
  <pre><code>df_mod['ER_mod']=(df_mod['Evap_mod']+np.mean(ddf['PET_PT']))/(ddf['PET_PT']+np.mean(ddf['PET_PT']))
  </code></pre>
  
  <p>Unfortunately, this calculation just returns NaN:</p>
  
  <pre><code>2003-12-20 00:30:00   NaN
  2003-12-21 00:30:00   NaN
  2003-12-22 00:30:00   NaN
  2003-12-23 00:30:00   NaN
  2003-12-24 00:30:00   NaN
  2003-12-25 00:30:00   NaN
  2003-12-26 00:30:00   NaN
  2003-12-27 00:30:00   NaN
  2003-12-28 00:30:00   NaN
  2003-12-29 00:30:00   NaN
  2003-12-30 00:30:00   NaN
  2003-12-31 00:30:00   NaN
  Name: ER_mod, Length: 729, dtype: float64
  </code></pre>
  
  <p>Does anyone has an idea why it returns NaN and how to overcome this issue?</p>