You're getting this error because you have written something like this:

   var t = (from e in E
            where e.column_one == condition_one
            join d in D on 
                 new {e.column_one, condition_two} equals
                 new {d.column_a, d.column_b}
            select new { e, d});

Instead of

   var t = (from e in E
            where e.column_one == condition_one
            join d in D on 
                 new {e.column_one, condition_two} equals
                 new {column_one = d.column_a, condition_two = d.column_b}
            select new { e, d});

Please help us improve by rating and commenting on this post.