title: Groupby Aggregate method is returning NaN always
body: <p>Hi I am running into this issue where my datasource events looks like this:</p>
  
  <pre><code>   event_id             device_id            timestamp  longitude  latitude
  0         1     29182687948017175  2016-05-01 00:55:25     121.38     31.24
  1         2  -6401643145415154744  2016-05-01 00:54:12     103.65     30.97
  2         3  -4833982096941402721  2016-05-01 00:08:05     106.60     29.7
  </code></pre>
  
  <p>I am trying to group the events by the device_id and then get the sum/mean/std of the variable over every event with that device_id:</p>
  
  <pre><code>events['latitude_mean'] = events.groupby(['device_id'])['latitude'].aggregate(np.sum)
  </code></pre>
  
  <p>But my Output is always:</p>
  
  <pre><code>event_id             device_id            timestamp  longitude  latitude
  0         1     29182687948017175  2016-05-01 00:55:25     121.38     31.24   
  1         2  -6401643145415154744  2016-05-01 00:54:12     103.65     30.97   
  2         3  -4833982096941402721  2016-05-01 00:08:05     106.60     29.70   
  3         4  -6815121365017318426  2016-05-01 00:06:40     104.27     23.28   
  4         5  -5373797595892518570  2016-05-01 00:07:18     115.88     28.66   
  
     latitude_mean  
  0            NaN  
  1            NaN  
  2            NaN  
  3            NaN  
  4            NaN
  </code></pre>
  
  <p>What am I doing wrong to keep getting the return value to be NaN for each row?</p>