title: Calculating a mean on groupby gives me a "DataError: No numeric types to aggregate"
body: <p>I'm trying to do some aggregations using groupby</p>
  
  <p>My code looks something like this:</p>
  
  <pre><code> Top15['est_pop'] = Top15['Energy Supply']/Top15['Energy Supply per Capita'] 
   new = Top15.groupby('Continent')['est_pop'].agg({'size': np.size, 'sum': 
                                   np.sum, 'mean': np.mean, 'std': np.std})
  </code></pre>
  
  <p>If I only have Size and sum in my agg, it works fine, but once I add mean or std, it gives me the following error:</p>
  
  <pre><code>DataError: No numeric types to aggregate 
  </code></pre>
  
  <p>I know "est_pop" is a float (and there are no empty values or anything weird like that)</p>
  
  <pre><code>type(Top15['est_pop'][(0)])
  
  ---&gt; float
  </code></pre>
  
  <p>Any idea what I'm doing wrong?  or what I can do to fix it?  </p>
  
  <p>Appreciate any help!</p>