title: Python groupby error, 'unhashable' Series object
body: <p>I have a Pandas DataFrame of values (floats) and country name strings ("UNITED STATES", "UNITED KINGDOM", etc).  I want to sum the values based on the countries:</p>
  
  <pre><code>Data['Values'].groupby(Data['Country']).sum()
  </code></pre>
  
  <p>Unfortunately, I get the following error:</p>
  
  <pre><code>TypeError: 'Series' objects are mutable, thus they cannot be hashed
  </code></pre>
  
  <p>Also, if I try to unique the Series consisting of the countries:</p>
  
  <pre><code>Data['Country'].unique()
  </code></pre>
  
  <p>I get the same error:</p>
  
  <pre><code>TypeError: 'Series' objects are mutable, thus they cannot be hashed
  </code></pre>
  
  <p>Why is this error occurring?  Is it something about the country names?  Any help would be greatly appreciated.  I am using Python 3.4 and Pandas 0.15.2.</p>