C# Core: No backing field could be found for property

Model:

public class User
{
    public long  Id { get; set; }
    [JsonIgnore]
    public LoginData LoginData { get; set; }        

    public User()
    {
        LoginData = new LoginData();
    }
};

[Owned]
public class LoginData
{
    public string Login { get; set; } = "";
    public string Password { get; set; } = "";

    public LoginData() { }
    public LoginData(string Login, string Password) 
    {
        this.Login = Login;
        this.Password = Password;
    }
}

OnModelCreating:

modelBuilder.Entity<User>().OwnsOne(p => p.LoginData, od => od.WithOwner()); 

In controller i try to use:

var users = await _context.User.Where(x => x.LoginData == loginData).ToListAsync();

And i get an error:

"No backing field could be found for property 'UserId' of entity type 'LoginData' and the property does not have a getter."

How can i fix it?