title: AttributeError: 'builtin_function_or_method' object has no attribute 'is_unique'
body: <p>I have the following data frame:</p>
  
  <p><strong>df1</strong></p>
  
  <pre><code>            NumOfTransactions  ComissionDeduction
  2011-01-10                  2               19.90
  2011-01-13                  2               19.90
  2011-01-26                  1                9.95
  </code></pre>
  
  <p><strong>df2</strong></p>
  
  <p>['2011-01-10']</p>
  
  <p>I need to join the two so I remain with  the row in df1 for when that date is also in df2.              </p>
  
  <pre><code> NumOfTransactions  ComissionDeduction
      2011-01-10                  2               19.90
  </code></pre>
  
  <p>I'm trying to achieve that functionality using:</p>
  
  <pre><code>impact = trades.index[trades.zero == total_columns].astype(str).tolist()
  trades_impact = transactions.join(impact)
  </code></pre>
  
  <p>However, I'm receiving the following error;<em>AttributeError: 'builtin_function_or_method' object has no attribute 'is_unique'</em></p>