The relationship could not be changed because one or more of the foreign-key properties is non-nullable

I am new to ASP.NET MVC, and I have this problem:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

Mi method in Juego.cs:

 public void UpdateJuegos(JuegoDTO juegosDTO)
        {
            using (var dbContext = new TrailersVideoJuegosEntities())
            {
                var juego = dbContext.Juego.FirstOrDefault(c => c.Id == juegosDTO.Id);
                if (juego != null)
                {
                    Mapper.Map(juegosDTO, juego);
                    Mapper.Map(juegosDTO.ListaImagenes, juego.Imagen);


                    if(juegosDTO.ListaImagenes != null)
                    {
                        foreach(var imagen in juegosDTO.ListaImagenes)
                        {
                            if(imagen.ImagenDeleted)
                            {
                                var deletedImg = juego.Imagen.FirstOrDefault(r => r.Id == imagen.Id);
                                if(deletedImg != null)
                                {
                                    juego.Imagen.Remove(deletedImg);

                                }
                            }
                        }
                    }
                    dbContext.SaveChanges();
                }
            }
        }

Help me please!!!