Error when trying to create a 'stub' course

using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;

namespace Treehouse.CodeChallenges
{
    public static class Repository
    {
        public static List<Course> GetCourses()
        {
            using (var context = new Context())
            {
                return context.Courses
                              .OrderBy(c => c.Teacher.LastName)
                              .ThenBy(c => c.Teacher.FirstName)
                              .ToList();
            }
        }

        public static List<Course> GetCoursesByTeacher(string lastName)
        {
            using (var context = new Context())
            {
                return context.Courses
                              .Where(c => c.Teacher.LastName == lastName)
                              .ToList();
            }
        }

        public static Course GetCourse(int id)
        {
            using (var context = new Context())
            {
                return context.Courses
                              .Include(c => c.Teacher)
                              .SingleOrDefault(c => c.Id == id);
            }
        }

        public static void AddCourse(Course course)
        {
            using (var context = new Context())
            {
                context.Courses.Add(course);
                context.SaveChanges();
            }
        }

        public static void UpdateCourse(Course course)
        {
            using (var context = new Context())
            {
                context.Courses.Attach(course);
                context.Entry(course).State = EntityState.Modified;
                context.SaveChanges();
            }
        }

        public static void DeleteCourse(int id)
        {
            using (var context = new Context())
            {
                // Create a "stub" course entity and set its Id property value to the passed in id parameter value
                var course = new Course() { Id == id };
                // Retrieve the course's entry using the context's Entry method and set its state to "Deleted"
                context.Entry(course).State = EntityState.Deleted;
                // Call the context's SaveChanges method to persist the changes to the database
                context.SaveChanges();
            }
        }
    }
}