title: Compass ignoring tint and shade functions in SCSS file
body: <p>I have an issue when compiling a few SCSS files using compass. Basically the <code>tint</code> and <code>shade</code> functions are being ignored. So, this:</p>
  
  <pre><code>border:solid 1px tint( $custom-ui-base-color, 5% );
  background-color:shade( $custom-ui-base-color, 15% );
  </code></pre>
  
  <p>becomes this:</p>
  
  <pre><code>border: solid 1px tint(#096fcb, 5%);
  background-color: shade(#096fcb, 15%);
  </code></pre>
  
  <p>Which obviously isn't valid CSS. Compass has replaced the variables but not calculated the functions, is there any reason why this might be the case? A setting or command line argument in compass maybe?</p>