Entity Framework 6 external framework gets null in dependent table

I am trying to use Primary Key Id from AspNetUsers for my Contractor table and it gives me the error "Cannot insert NULL value into Column Id". The error occurs when I try to create a Contractor after user registration and they appear in AspNetUsers.

  public class Contractor
{
    [Key, ForeignKey("ApplicationUser")]
    public string ID { get; set; }

    public virtual ApplicationUser ApplicationUser { get; set; } 



 public class ApplicationUser : IdentityUser
{

    public virtual Contractor Contractor { get; set; }

      

        
        
        
      

    

EDIT

The Contractor is the base class for the musician. Code for creating a musician from a controller

     public ActionResult Create([Bind(Include = "FirstName,LastName,ZipCode,Phone,PrimaryInstrument,PrimaryGenre,NextDateAvailable,WebsiteLink, YouTubeLink,SoundCloudLink, ReverbNationLink,YearsOfExperience,Description")] Musician musician, HttpPostedFileBase image)
    {
        if (ModelState.IsValid)
        {
            if (image != null)
            {
                string imageFilePath = Server.MapPath("~/Images/");
                image.SaveAs(imageFilePath + image.FileName);

                musician.ImageData = new byte[image.ContentLength];
                musician.ImageMimeType = image.ContentType;
                musician.ImageName = image.FileName;

            }


            musician.CreateDate = DateTime.Now;

            db.Musicians.Add(musician);
            db.SaveChanges();
            return View();
        }

        return View(musician);
    }

      

        
        
        
      

    

Musician class:

  public class Musician : Contractor
{
    public string PrimaryInstrument { get; set; }
    public string PrimaryGenre { get; set; }
    public string WebsiteLink { get; set; }
    public string YouTubeLink { get; set; }
    public string SoundCloudLink { get; set; }
    public string  ReverbNationLink { get; set; }
}

      

        
        
        
      

    

}