title: Pandas Mask query
body: <p>I have a query in masks - My X is defined as below</p>
  
  <pre><code>X = pd.DataFrame({'a':[1,250,480],
                 'b':[60,51,101],
                 'c':[15,689,1],
                 'd':[1,1,1],
                 'e':[2,2,2]})
  </code></pre>
  
  <p>which gives my X as below</p>
  
  <pre><code>    a   b   c   d   e
  0   1   60  15  1   2
  1   250 51  689 1   2
  2   480 101 1   1   2
  </code></pre>
  
  <p>When I create a mask as
   <code>x['d'] =x.mask(x['c'] &gt;100, "b")</code> </p>
  
  <p>I get the below output..</p>
  
  <pre><code>    a   b   c   d   e
  0   1   60  15  1   2
  1   250 51  689 b   2
  2   480 101 1   480 2
  </code></pre>
  
  <p>I don't understand where the 480 in column d is coming from.</p>
  
  <p>Specifically x['d'] is now [1 b 480]
  The b is clear, but where is 480 coming from ?? I think i am missing something about how masks work</p>
  
  <p>Thanks.</p>