title: Pandas error with multidimensional key using .loc and a boolean
body: <p>Been running into this same error for 2 weeks, even though the code worked before. Not sure if I updated pandas as part of another library install, and maybe something changed there. Currently on version 23.4. Expected outcome is returning just the row with that identifier value. </p>
  
  <pre><code>In [42]: df.head()
  
  Out[43]:
  
  index   Identifier  ... 
  0        51384710   ... 
  1        74838J10   ... 
  2        80589M10   ...     
  3        67104410   ... 
  4        50241310   ... 
  
  [5 rows x 14 columns]
  In [43]: df.loc[df.Identifier.isin(['51384710'])].head()
      Traceback (most recent call last):
        File "C:\anaconda3\lib\site-packages\IPython\core\interactiveshell.py", line 2862, in run_code
          exec(code_obj, self.user_global_ns, self.user_ns)
  File "&lt;ipython-input-44-a3dbf43451ef&gt;", line 1, in &lt;module&gt;
       df.loc[df.Identifier.isin(['51384710'])].head()
  File "C:\anaconda3\lib\site-packages\pandas\core\indexing.py", line 1478, in __getitem__
          return self._getitem_axis(maybe_callable, axis=axis)
  File "C:\anaconda3\lib\site-packages\pandas\core\indexing.py", line 1899, in _getitem_axis
          raise ValueError('Cannot index with multidimensional key')
      **ValueError: Cannot index with multidimensional key**
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/vu0VY.png" rel="nofollow noreferrer">Code Snippet</a></p>