title: Why is mix() not working with my colors in LESSC?
body: <p>My less file piggy-backs on to the end of the Twitter Bootstrap files (so things like <code>@white</code> are defined already.  Here is my less code:</p>
  
  <pre><code>@columnSortScreen: #99f;
  // the color used
  @columnSort: mix( @white, @columnSortScreen );
  </code></pre>
  
  <p>When compiled, I get the following error (everything underlined by carets is highlighted):</p>
  
  <pre><code>non_object_property_loadError: error evaluating function `mix`: Cannot read property 'value' of undefined in /Users/cmv/Sites/...../tablesorter.less:12:14
  11 // the color used
  12  @columnSort: mix( @white, @columnSortScreen );
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  </code></pre>
  
  <p>What am I doing wrong?  Lessc is version 1.3.0.</p>
  
  <p>Thanks!</p>