Undescriptive exception when calling fillna() with invalid method
## In [235]: data.geq.fillna(method='ffil')

Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "C:\Python27\lib\site-packages\pandas\core\series.py", line 2051, in fillna
    new_values = self.values.take(indexer)
UnboundLocalError: local variable 'indexer' referenced before assignment
