Coudn't save many-to-many code first db ?

I'm using code first and fluent api this is my models

public Admin()
        {
            this.course = new HashSet<Courses>();
        }
        [Key]
        public int ID { get; set; }
   public string LoginName { get; set; }
   public virtual ICollection<Courses> course { get; set; }
}

 public Courses()
            {
                this.admin = new HashSet<Admin>();
            }
            [Key]

            public int ID { get; set; }
      public string Name { get; set; }
     public virtual ICollection<Admin> admin { get; set; }
    }
and this is fluent api

modelBuilder.Entity<Admin>()
                .HasMany(e => e.course)
                .WithMany(e => e.admin);
and this is my controller which I'm getting the courseid from cshml

public ActionResult Admins( Admin rec, IList CourseId) {

      foreach (var item in CourseId)
        {
            var cr = new Courses();
            cr.ID = item;
            rec.course.Add(cr);
 }
  db.Admins.Add(rec);
  db.SaveChanges();
it gives me error when I'm trying to save db.SaveChanges()

could any one tell me what's the wrong here ?