JQuery Calculation returns NaN
<p>I'm using <a href="http://www.pengoworks.com/workshop/jquery/calculation/calculation.plugin.htm" rel="nofollow">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>
