Linq To SQL. why I am getting Error The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'Join'

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LINQ
{
    class Program
    {
        static void Main(string[] args)
        {
            NorthwindEntities linq = new NorthwindEntities();
            IEnumerable<employee> emps=    linq.Employees.Where(x => x.TitleOfCourtesy == "Ms." || x.Title == "Sales Representative");

            IEnumerable<customer> csts = linq.Customers.Where(x => x.Country == "Mexico");
            var v = from l1 in emps
                    join l2 in csts
                        on l1.EmployeeID equals l2.CustomerID
                    select l1.Title;           
        
        }
    }
}