ASP.NET Core 3.0 MVC update model with list objects - add object

I have the following classes:

public class Invoice
{    
     [key]
     public Guid InvoiceID { get; set; }
     public string Name { get; set; }
     public List<Invoice_item> Invoice_items { get; set; }
     
     public Invoice()
     {
         InvoiceID = Guid.NewGuid();
         Invoice_items = new List<Invoice_item>();
     }
} 

and:

public class Invoice_item
{
     [Key]
     public Guid Invoice_itemID { get; set; }
     public string Description{ get; set; }
     public int Price { get; set; }
     public int qty { get; set; }
     
     public Invoice_item()
     {
            Invoice_itemID = Guid.NewGuid();
     }
}

It works fine. I can edit this model. Controller: ...

Invoice invoice = await _context.Invoice.Include(i => i.Invoice_items).Where(x => x.InvoiceID == id).FirstOrDefaultAsync();

invoice.Name = "2020/122";
invoice.Invoice_items[0].Description = "Some name";
invoice.Invoice_items[0].Price = 1;
invoice.Invoice_items[0].qty = 1;

_context.Update(invoice);
await _context.SaveChangesAsync();

// WORK OK

But the problem is when I want to add an item to the list: Controller: ...

Invoice invoice = await _context.Invoice.Include(i => i.Invoice_items).Where(x => x.InvoiceID == id).FirstOrDefaultAsync();

invoice.Name = "2020/122";
invoice.Invoice_items[0].Description = "Description 1";
invoice.Invoice_items[0].Price = 1;
invoice.Invoice_items[0].qty 1;

Invoice_item item = new Invoice_item()
{
  Description = "Description 2",
  Price = 2,
  qty = 2
};

invoice.Invoice_items.Add(item);   // PROBLEM
                       
_context.Update(invoice);
await _context.SaveChangesAsync();

When editing the "Invoice" record, it is possible to edit all elements of the record and the elements of the "Invoice_item" list attached to it. The problem occurs when I want to add an item to the "Invoice_item" list attached to the "Invoice" record.

Please help.