I don't understand why I have a compiler error.

namespace Treehouse.CodeChallenges
{
    public class Course
    {
        public int Id { get; set; }
        public int TeacherId { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }
        public int Length { get; set; }

        public Teacher Teacher { get; set; }
    }
}

using System.Collections.Generic;

namespace Treehouse.CodeChallenges
{
    public class Teacher
    {
        public Courses ()
        {
            Courses = new List<Course>();
        }

        public int Id { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public ICollection<Course> Courses { get; set; }
    }
}