.net mvc2 join query issue

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here i got a error "The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'join'"

please solve this

viewModel = (from r in ReleaseDB.releases
join p in ReleaseDB.projects on r.ProjectID equals p.ProjectId
join q in ReleaseDB.platforms on p.Platform_Proj equals q.Name
group new { r, p, q } by new { r.EHCUCount, r.ReleaseID, r.ReleaseName, r.Revision, p.ProjectName, r.Disposition,q.Name, r.IsSubmitted, r.UserID } into grp
select new IndexRelease
{
ReleaseID = grp.Key.ReleaseID,
ReleaseName = grp.Key.ReleaseName,
Revision = grp.Key.Revision,
Project = grp.Key.ProjectName,
Platform = grp.Key.Name,
Disposition = grp.Key.Disposition,
EHCUCount = grp.Key.EHCUCount,
IsSubmited = grp.Key.IsSubmitted,
UserID = grp.Key.UserID
}).ToList();

break;