transform in groupby throws TypeError when run with python -O option
I have a script that works fine when run without any options but generates the following traceback when run with 'python -O':

  File "C:\Python27\lib\site-packages\pandas\core\groupby.py", line 1745, in transform
    return self._transform_item_by_item(obj, wrapper)
  File "C:\Python27\lib\site-packages\pandas\core\groupby.py", line 1777, in _transform_item_by_item
    raise TypeError('Transform function invalid for data types')
TypeError: Transform function invalid for data types

I have Python 2.7.3 and pandas 0.9.0:

Python 2.7.3 (default, Apr 10 2012, 23:24:47) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

> > > import pandas
> > > pandas.**version**
> > > '0.9.0'
