linq sql join gets null with query syntax

Question

in my asp.net core mvc 3 project, when I use method syntax, the department name appears in the table on index view, but when I use the query syntax, the department name does not appear. On debugging course.Department comes null. I can't figure out what is my fault?

Course and Department model

public class Course
    {
        [Key]
        public int CourseId { get; set; }
        public string CourseName { get; set; }
        public int Credits { get; set; }

        public int DepartmentId { get; set; }
        public Department Department { get; set; }
    }

public class Department
    {       
        public int DepartmentId { get; set; }
        public string DepartmentName { get; set; }
        public decimal Budget { get; set; }
        public ICollection<Course> Courses { get; set; }
    }

CourseController

public IActionResult Index()
        {
            //method syntax
            //var courses = _context.Courses.Include(x=>x.Department).ToList();

            //query syntax
            var courses = from course in _context.Courses
                          join dept in _context.Departments on course.DepartmentId equals dept.DepartmentId
                          select course;


            return View(courses);
        }

index.cshtml

.....
<tbody>
@foreach (var item in Model) {
        <tr>
            <td>
                @Html.DisplayFor(modelItem => item.CourseId)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.CourseName)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Credits)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Department.DepartmentName)
            </td>
       </tr>
....