title: Bug in pandas query() method?
body: <p>I was experimenting several use cases for the pandas query() method, and tried one argument that threw an exception, but yet caused an unwanted modification to the data in my DataFrame.</p>
  
  <pre><code>In [549]: syn_fmax_sort
  Out[549]: 
       build_number      name    fmax
  0             390     adpcm  143.45
  1             390       aes  309.60
  2             390     dfadd  241.02
  3             390     dfdiv   10.80
  ....
  211           413     dfmul  215.98
  212           413     dfsin   11.94
  213           413       gsm  194.70
  214           413      jpeg  197.75
  215           413      mips  202.39
  216           413     mpeg2  291.29
  217           413       sha  243.19
  
  [218 rows x 3 columns]
  </code></pre>
  
  <p>So I wanted to use <code>query()</code> to just take out a subset of this dataframe that contains all the <code>build_number</code> of 392, so I tried:</p>
  
  <pre><code>In [550]: syn_fmax_sort.query('build_number = 392')
  </code></pre>
  
  <p>That threw a <code>ValueError: cannot label index with a null key</code> exception, but not only that, it returned back the full dataframe to me,and caused all the <code>build_number</code> to be set to 392:</p>
  
  <pre><code>In [551]: syn_fmax_sort
  Out[551]: 
       build_number      name    fmax
  0             392     adpcm  143.45
  1             392       aes  309.60
  2             392     dfadd  241.02
  3             392     dfdiv   10.80
  ....
  211           392     dfmul  215.98
  212           392     dfsin   11.94
  213           392       gsm  194.70
  214           392      jpeg  197.75
  215           392      mips  202.39
  216           392     mpeg2  291.29
  217           392       sha  243.19
  
  [218 rows x 3 columns]
  </code></pre>
  
  <p>However, I have since figured out how to get value 392 only, if I used <code>syn_fmax_sort.query('391 &lt; build_number &lt; 393')</code>, it works/</p>
  
  <p>So my question is: Is the behavior that I observed above when I queried the dataframe wrongly due to a bug in the <code>query()</code> method?</p>