jQuery Uncaught TypeError：无法读取未定义的属性“ toString”

Question

我在C＃Razor视图中使用以下jQuery函数：

function calculateTotal(cost, freight, tax) {
   var costNum = cost.toString();
   costNum = costNum.replace(/,/g, '');
   costNum = parseFloat(costNum);

   var freightNum = freight.toString(); 
   freightNum = freightNum.replace(/,/g, '');
   freightNum = parseFloat(freightNum);

   var taxNum = tax.toString();
   taxNum = taxNum.replace(/,/g, '');
   taxNum = parseFloat(taxNum);

   var total = 0;
   total = costNum + freightNum + taxNum;

   total = numberWithCommas(total);

   return total;
}

并从多个地方调用它：

var MonthlyGrandTotal = calculateTotal(totalMonthCost, totalMonthFreight, totalMonthTax);

当我在浏览器中加载页面时，在控制台窗口中看到“未捕获的TypeError：无法读取未定义的属性'toString'”。它似乎并没有真正影响任何东西，但是我很好奇为什么看到错误。