在Linq查询中使用列表时出现空引用错误

Question

我通常在Linq Pad中编写测试代码，然后将其添加到我的MVC应用程序中。以下代码适用于Linq Pad，但Visual Studio会抛出空引用异常。

var first =
(from a in AdviceLineCalls
where a.CallDate != null
orderby a.CallDate
select a.CallDate).First();


var last =
(from a in AdviceLineCalls
where a.CallDate != null
orderby  a.CallDate descending
select a.CallDate).First();

var calls =
(from a in AdviceLineCalls
join s in Staff on a.StaffID equals s.StaffID
where a.CallLength != null
&& s.Employed == true
select new Call()
{
    Staff = s.LastName,
    Minutes = a.CallLength.Value,
    CallDate = a.CallDate
}).ToList();

var whoCalled =
from c in calls
where (c.CallDate >= first && c.CallDate <= last)
group c by c.Staff into grp
let count = grp.Count()
let mins = grp.Sum (g => g.Minutes)
let avg = grp.Average (g => g.Minutes)
select new WhoCalled() 
{Status = grp.Key, Frequency = count, Minutes = mins, Avg = avg};

在Visual Studio中，我在who.called on c.CallDate上的where子句中得到一个空引用错误：对象引用未设置为对象的实例。我不确定为什么这可以在Linq Pad中工作，但不能在Visual Studio中工作。