title: Incompatible indexer with Series
body: <p>Why do I get an error:</p>
  
  <pre><code>import pandas as pd
  a = pd.Series(index=[4,5,6], data=0)
  print a.loc[4:5]
  a.loc[4:5] += 1
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>4    0
  5    0
  Traceback (most recent call last):
    File "temp1.py", line 9, in &lt;module&gt;
  dtype: int64
      a.loc[4:5] += 1
    File "lib\site-packages\pandas\core\indexing.py", line 88, in __setitem__
      self._setitem_with_indexer(indexer, value)
    File "lib\site-packages\pandas\core\indexing.py", line 177, in _setitem_with_indexer
      value = self._align_series(indexer, value)
    File "lib\site-packages\pandas\core\indexing.py", line 206, in _align_series
      raise ValueError('Incompatible indexer with Series')
  ValueError: Incompatible indexer with Series
  </code></pre>
  
  <p>Pandas 0.12.</p>