I’m trying to join two tables using linq to sql. I am getting error on join keyword that "Error 14 The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'Join'."

var query = (from es in Company.Employee_Salary
                         join ed in Company.Employee_Details on es.Employee_ID equals es.Student_ID
                         orderby es.Employee_ID
                         select new {  es.Month }).ToList();