title: key error while grouping for a present key
body: <p>I am trying to execute this - 
  but it gives a key error for site_id while site_id is present in my dataset.
  I have double checked the spellings but this problem still persists.Traceback is exceptionally long to paste here.</p>
  
  <pre><code>air_temperature_filler = 
  pd.DataFrame(weather_df.groupby(['site_id','day','month']) 
  ['air_temperature'].mean(),columns=["air_temperature"])
  weather_df.update(air_temperature_filler,overwrite=False)
  </code></pre>