ef core one to many relationship throw exception Cannot add or update a child row

My Shop and Product entities have a one to many relationship, please see my models

public class Product
{
   public int ID { get; set; }
   public string Name { get; set; }
   public string Tag { get; set; }
   public string Brand { get; set; }
   public string Place { get; set; }

   public int ShopID { get; set; }
   public Shop Shop { get; set; }
}

public class Shop
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string Comment { get; set; }
    public string Number { get; set; }

    public ICollection<Product> Products { get; set; }
}



 public class ShopContext : DbContext
    {
        public ShopContext(DbContextOptions<ShopContext> options) : base(options)
        {
        }

        public DbSet<Shop> Shops { get; set; }
        public DbSet<Product> Products { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Shop>().ToTable("shop");
            modelBuilder.Entity<Product>().ToTable("product");
        }

    }

public class CreateModel : PageModel
{
    private readonly ShopContext _context;

    public CreateModel(ShopContext context)
    {
        _context = context;
    }

    public IActionResult OnGet()
    {
        ViewData["Shop"] = new SelectList(_context.Shops, "ID", "Name");

        return Page();
    }

    [BindProperty]
    public Product Product { get; set; }

    public async Task<IActionResult> OnPostAsync()
    {
        if (!ModelState.IsValid)
        {
            return Page();
        }

        _context.Products.Add(Product);
        await _context.SaveChangesAsync();

        return RedirectToPage("./Index");
    }
}

The shop in create page is a dropdown list, when I post the new product to the handler, I can see the product.ShopID is an ID which exist in Shop table, and the product.Shop.ID as same as the product.ShopID. But when I call saveChangesAsync(), it throws an exception:

MySqlException: Cannot add or update a child row: a foreign key constraint fails (shopdb.product, CONSTRAINT FK_product_shop_ShopID FOREIGN KEY (ShopID) REFERENCES shop (ID) ON DELETE CASCADE)

I tried to set product.Shop to null before call saveChangesAsync method, but still doesn't work, anyone can help?

ASP.NET Core 2.0 Razor Pages Application, Nuget: Pomelo.EntityFrameworkCore.MySql v2.0.1