NotSupportedException with linq request and groupBy

I get a "NotSupportedException" when I run this Linq request. "The specified method is not supported." Which method? Even when "Count ()" is commented out, the error is the same. Null Inner Exception.

Stack trace:

in Hydra.WPF.ViewModels.AddressListViewModel._primaryMemberListWorker_DoWork (sender object, DoWorkEventArgs e) in M: \ Projects \ Hydra \ WPF \ ViewModels \ AddressList \ AddressListViewModel.cs: line 1377 in the System.ComponentModelWorkground.Background

SQL Query:

Select * From _Members
Inner Join AddressDetailsCCN a on a.MemberId=_Members.MemberID
Inner Join 
  (
  Select Address+Address2 as CombinedAddress 
  From AddressDetailsCCN Where ListId=84 
  group by Address+Address2
  Having COUNT(*)>1
  ) B on B.CombinedAddress=A.Address+A.Address2
Where CombinedAddress is not null AND CombinedAddress!='' AND a.ListId=84
Order by ClientID, CombinedAddress

Linq:

var grouped = 
   (from mem in session.Query<Member>()
    join detail in session.Query<Detail>() on mem.Id equals detail.Member.Id
    join d2 in (from d3 in session.Query<Detail>()
                where d3.AddressList.Id == criteria.AddressList.Id
                group d3 by d3.Address + d3.Address2 into x
                where x.Count() > 1
                select x) on detail.Address+detail.Address2 equals d2.Key
    where detail.AddressList.Id==criteria.AddressList.Id
    select mem);