title: No numeric types to aggregate - change in groupby() behaviour?
body: <p>I have a problem with some groupy code which I'm quite sure once ran (on an older pandas version). On 0.9, I get <em>No numeric types to aggregate</em> errors. Any ideas?</p>
  
  <pre><code>In [31]: data
  Out[31]: 
  &lt;class 'pandas.core.frame.DataFrame'&gt;
  DatetimeIndex: 2557 entries, 2004-01-01 00:00:00 to 2010-12-31 00:00:00
  Freq: &lt;1 DateOffset&gt;
  Columns: 360 entries, -89.75 to 89.75
  dtypes: object(360)
  
  In [32]: latedges = linspace(-90., 90., 73)
  
  In [33]: lats_new = linspace(-87.5, 87.5, 72)
  
  In [34]: def _get_gridbox_label(x, bins, labels):
     ....:             return labels[searchsorted(bins, x) - 1]
     ....: 
  
  In [35]: lat_bucket = lambda x: _get_gridbox_label(x, latedges, lats_new)
  
  In [36]: data.T.groupby(lat_bucket).mean()
  ---------------------------------------------------------------------------
  DataError                                 Traceback (most recent call last)
  &lt;ipython-input-36-ed9c538ac526&gt; in &lt;module&gt;()
  ----&gt; 1 data.T.groupby(lat_bucket).mean()
  
  /usr/lib/python2.7/site-packages/pandas/core/groupby.py in mean(self)
      295         """
      296         try:
  --&gt; 297             return self._cython_agg_general('mean')
      298         except DataError:
      299             raise
  
  /usr/lib/python2.7/site-packages/pandas/core/groupby.py in _cython_agg_general(self, how, numeric_only)
     1415 
     1416     def _cython_agg_general(self, how, numeric_only=True):
  -&gt; 1417         new_blocks = self._cython_agg_blocks(how, numeric_only=numeric_only)
     1418         return self._wrap_agged_blocks(new_blocks)
     1419 
  
  /usr/lib/python2.7/site-packages/pandas/core/groupby.py in _cython_agg_blocks(self, how, numeric_only)
     1455 
     1456         if len(new_blocks) == 0:
  -&gt; 1457             raise DataError('No numeric types to aggregate')
     1458 
     1459         return new_blocks
  
  DataError: No numeric types to aggregate
  </code></pre>