Error decompile LINQ syntax...

I used version 7.1.0.143 ...but why this code cannot resolve by refelector..Pls help me...

private FileStorageFolder GetCapsuleContentFolder(FileStorageDataContext db)
{
    IQueryable<FileStorageFolder> source = from dbFolder in db.FileStorageFolders
        where (dbFolder.ParentID == null) && (dbFolder.Purpose == reportFolderPurpose)
        select dbFolder into dbFolder
        join dbSubFolder in db.FileStorageFolders on dbFolder.ID equals dbSubFolder.ParentID into dbSubFolder
        where (dbSubFolder.Purpose == capsulelayoutFolderPurpose) && (dbSubFolder.FolderName == capsuleReportContent)
        select dbSubFolder;
    Instrument.Assert(source.Count<FileStorageFolder>() == 1);
    return source.Single<FileStorageFolder>();
}

 

error on second "where" syntax...("Ambigous Invocation")