Only primitive types or enumeration types are supported in this context- linq join

Question

I am getting this error when I try to join

var users = _users.Get();
var userApprovals =
(from approval in _entities.ApprovalEntities
join userDetail in users on approval.UserKey equals userDetail.UserId
 where approval.EmployeeUid == employeeUid
 select new UserApproval
{
Id = approval.Id,
EmployeeUid = approval.EmployeeUid,
UserKey = approval.UserKey,
UserId = approval.UserId,
UserName = userDetail.FirstName + " " + userDetail.LastName
}).ToList();

error Only primitive types or enumeration types are supported in this context

Thanks

Answer 1

fixed, var userApprovals =

(from approval in _entities.ApprovalEntities.ToList()
join userDetail in users on approval.UserKey equals userDetail.UserId
 where approval.EmployeeUid == employeeUid
 select new UserApproval
{
Id = approval.Id,
EmployeeUid = approval.EmployeeUid,
UserKey = approval.UserKey,
UserId = approval.UserId,
UserName = userDetail.FirstName + " " + userDetail.LastName
}).ToList();