title: Python pandas groupby key error in pandas.hashtable.PyObjectHashTable.get_item
body: <p>I am doing what seems to be a simple group by in Pandas. The column is a string column with no NaN's or weird strings. However, I keep getting the below error. Does anyone know why this mights happen? I feel like it may have something to do with my data, but it all seems to be ok...</p>
  
  <p>I am running <code>by_user = df.groupby('User')</code></p>
  
  <p>and the stack trace:</p>
  
  <pre><code>by_user = df.groupby('User')
  File "c:\Anaconda\lib\site-packages\pandas\core\generic.py", line 2773, in groupby
  sort=sort, group_keys=group_keys, squeeze=squeeze)
  File "c:\Anaconda\lib\site-packages\pandas\core\groupby.py", line 1142, in groupby
  return klass(obj, by, **kwds)
  File "c:\Anaconda\lib\site-packages\pandas\core\groupby.py", line 388, in __init__ level=level, sort=sort)
  File "c:\Anaconda\lib\site-packages\pandas\core\groupby.py", line 2041, in _get_grouper
  gpr = obj[gpr]
  File "c:\Anaconda\lib\site-packages\pandas\core\frame.py", line 1678, in __getitem__
  return self._getitem_column(key)
  File "c:\Anaconda\lib\site-packages\pandas\core\frame.py", line 1685, in _get      item_column
  return self._get_item_cache(key)
  File "c:\Anaconda\lib\site-packages\pandas\core\generic.py", line 1052, in _ge
  t_item_cache
  values = self._data.get(item)
  File "c:\Anaconda\lib\site-packages\pandas\core\internals.py", line 2565, in get
  loc = self.items.get_loc(item)
  File "c:\Anaconda\lib\site-packages\pandas\core\index.py", line 1181, in get_loc
  return self._engine.get_loc(_values_from_object(key))
  File "index.pyx", line 129, in pandas.index.IndexEngine.get_loc (pandas\index.
  c:3656)
  File "index.pyx", line 149, in pandas.index.IndexEngine.get_loc (pandas\index.
  c:3534)
  File "hashtable.pyx", line 696, in pandas.hashtable.PyObjectHashTable.get_item
  (pandas\hashtable.c:11911)
  File "hashtable.pyx", line 704, in pandas.hashtable.PyObjectHashTable.get_item
  (pandas\hashtable.c:11864)
  KeyError: 'User'
  </code></pre>
  
  <p><code>df.info():</code></p>
  
  <pre><code>User Code        175167 non-null object
  Version          175167 non-null object
  Date Accessed    175167 non-null datetime64[ns]
  Series           175167 non-null object
  Software         175167 non-null object
  User             175167 non-null object
  </code></pre>