Error Entering Data (db.SaveChanges ()) Entity Framework ASP.NET MVC

I have a problem I can not solve, so I am posting to the teachers to try to help me ... thanks

These are my data model

public partial class Cadastro
{
    [Key, Column(Order = 0)]
    public int cd_cadastro { get; set; }
    public string razao  { get; set; }

    public virtual Cliente Cliente { get; set; }
    public virtual Vendedor Vendedor { get; set; }
}

public partial class Cliente
{
    [Key, Column(Order = 0)]
    public int cd_cadastro { get; set; }
    public string operacao_fiscal  { get; set; }
}

public partial class Vendedor
{
    [Key, Column(Order = 0)]
    public int cd_cadastro { get; set; }
    public decimal pr_comissao  { get; set; }
}

--- relationship in Entities

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Cadastro>().HasKey(s => new { s.cd_cadastro });

    modelBuilder.Entity<Cadastro>().HasRequired(a => a.Cliente).WithOptional().WillCascadeOnDelete(true);
    modelBuilder.Entity<Cadastro>().HasRequired(a => a.Vendedor).WithOptional().WillCascadeOnDelete(true);
}

So I created a class, because the user can include the information in a single view

public partial class CadCli
{
    public virtual Cadastro Cadastro { get; set; }
    public virtual Cliente Cliente { get; set; }
    public virtual Vendedor Vendedor { get; set; }
}

In my view create (GET) is well @model CadCli

My Controller in this way:

[HttpPost]
public ActionResult Create (Cadastro cadastro, Cliente cliente, Vendedor vendedor)
{
    // All fields of registration , usually comes filled

    // Here is valid
    if ( ModelState.IsValid )
    {
        cadastro.cd_cadastro = 1; // Test

        db.Cadastro.Add ( cadastro) ;
                
        // Does not come here because this FALSE
        if ( cadastro.ind_cliente )
        {
            cliente.cd_cadastro = cadastro.cd_cadastro ;
            db.Cliente.Add (client ) ;
        }

        // Here is the problem
        db.SaveChanges ();

        // Even without entering the line ind_cliente
        // It does insert the cadastro ( OK )
        // but makes an insert cliente
        // And it should not , because I did not add
        // look the line ind_cliente is SET TO FALSE

        // Seems to me that would have to have something
        // " Do not add the client in this CadCli "
    }

In this case should not make the insert CLIENTE because it was not marked as a CLIENTE and not entered in line ind_cliente

But when ind_cliente line is true, it is normal This also happens with VENDEDOR or any other class within CadCli

Please how can I solve this without having to change my data model?