LINQ query joining Account, Case and Activity entities

Hello,

I try to make the following query using Account, Case and Activity entities :

var findCases =
from a in context.AccountSet
join c in context.IncidentSet
on a.AccountId equals c.CustomerId.Id
join p in context.ActivityPointerSet
on c.Id equals p.RegardingObjectId.Id
select new 
{
TicketNumber = c.TicketNumber,
IncidentId = c.IncidentId,
Title = c.Title,
CreatedOn = c.CreatedOn,
StateCode = c.StateCode,
AccountName = a.Name,
};

The execution of this query gives the error:

AttributeFrom and AttributeTo must be either both specified or both ommited. You can not pass only one or the other. AttributeFrom: , AttributeTo: regardingobjectid 

Any suggestion will be appreciated.

Regards.

Radu Antonache