加两个小数？ LINQ结果在C＃中

Question

var total= jobs.Where(x => x.num == pNum).Sum(x => x.amount);

var totalTwo= person.Where(x => x.num == pNum).Sum(x => x.amount);

我想添加两个LINQ查询的结果。如果一个为null，则结果需要将firs total的空/空字符串值添加到第二个total

if (total== null)
    total = "0";
ViewBag.OverallTotal = total += totalTwo;

ViewBag抛出无效的参数错误