title: JQuery Calculation returns NaN
body: <p>I'm using <a href="http://www.pengoworks.com/workshop/jquery/calculation/calculation.plugin.htm" rel="nofollow noreferrer">JQuery Calculation plugin</a> with the following script:</p>
  
  <pre><code>function recalc() {
           jQuery("input[class^=percent]").calc(
              "(invoice - cost)/invoice * 100",
              {
                  invoice: jQuery("input[class^=invoice]"),
                  cost: jQuery("input[class^=cost]")
              },
                  function(s) {
                  // return the number as a perrcent amount
                return s.toFixed(2) + "%";
              }
          )
      };
  </code></pre>
  
  <p>Just wondering how to stop "NaN%" being displayed when the calculation is invalid (eg. when dividing by zero) and display an empty string instead.</p>
  
  <p>Greatly appreciate any help. Cheers!</p>