linq to ef中的“first”操作出现意外异常

我正在尝试获取数据库中具有给定“userguid”的第一个项,但“first”扩展方法引发以下异常: System.InvalidOperationException: Sequence contains no elements

以下是一些什么有效,什么无效的例子:

// Works
var FoundUser1 = MyEntities.Users.First();

// Works
var FoundUser3 = MyEntities.Users.ToList().First(r => r.UserGuid == SampleUserGuid);

// Throws System.InvalidOperationException: Sequence contains no elements.
var FoundUser2 = MyEntities.Users.First(r => r.UserGuid == SampleUserGuid);

有人有什么想法吗?

编辑

更奇怪的代码示例:

// UserList1 is empty
var Query1 = from x in MyEntities.Users
             where x.UserGuid == criteria.Value
             select x;
var UserList1 = Query1.ToList();


// UserList2 has 3 users, including one with a 
// matching guid value.
var Query2 = from x in MyEntities.Users
             select x;
var UserList2 = Query2.ToList();  
var Query22 = from x in Query2
              where x.UserGuid == criteria.Value
              select x;
var UserList22 = Query22.ToList();

// Works
User FoundUser = null;
foreach (var ThisUser in MyEntities.Users)
{
   if (ThisUser.UserGuid == criteria.Value)
      FoundUser = ThisUser;
}