title: Pandas unhashable type: 'numpy.ndarray' with pandas groupby
body: <p>I am stuck on probably an elementary problem with pandas DataFrame. In the following code snippet, I insert a calculated column 'CAPACITY_CHECK' then I try to group by the data based on it. But I keep having the following error: TypeError: unhashable type: 'numpy.ndarray'</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>TEMP['CAPACITY_CHECK'] = TEMP[['ADD_CAPACITY_ST', 'CAPACITY_ST', 'VOLUME_PER_SUPPLIER']].apply(lambda X: numpy.where(X[0]+X[1]&lt;X[2],'Non OK', 'OK'), axis=1)
  TEMP.groupby('CAPACITY_CHECK')['ID'].count()</code></pre>
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  </div>
  </p>
  
  <p>Since I am not trying to modify any immutable object and the new column's type is "Series", I don't understand why I am having the error. </p>
  
  <p>Thanks in advance</p>