title: Pandas groupby.mean() giving me "No numeric types to aggregate" - but .sum() works
body: <p>I'm working in Pandas in Python 3. For some reason, I can group by and then sum() my dataframe:</p>
  
  <pre><code>full_data.groupby('polarity')['pos'].sum()
  
  polarity
  both      1.842
  neg       5.241
  neu     496.026
  pos     245.105
  Name: pos, dtype: float64
  </code></pre>
  
  <p>When I swap out <em>mean</em> for <em>sum</em>, though, I get this error:</p>
  
  <pre><code>DataError: No numeric types to aggregate
  </code></pre>
  
  <p>Do you know why this would be the case? I've already confirmed that every item in the 'pos' column is a float, running the following code with no output:</p>
  
  <pre><code>for i in full_data.loc[:,'pos']:
  if type(i) != float:
      print('not a float')
  </code></pre>
  
  <p>Thanks for your help!</p>