i am trying to make a project for my school. I am trying to read a table where some null values are in an integer column. I've made it an integer but i can't get the null values into the integer.

I've already searched in stackoverflow but none of the answers i could make at my project. Can someone provide me some help, tell me where i have to put the code to make it work again. I just started as programmer. 

This is my database conn string + reader: 

```
        public static List<Klant> GetAlleklanten()
        {
            var result = new List<Klant>();
            using (var conn = new SqlConnection(ConnectionString))
            {
                conn.Open();
                const string query = "select k.klantid, k.naam, k.adres, k.telefoonnummer, k.woonplaats, k.email, k.wachtwoord, kp.klantpasid from klant k left join klantpas kp on k.klantid = kp.klantid";
                SqlCommand selectKamers = new SqlCommand(query, conn);
                SqlDataReader reader = selectKamers.ExecuteReader();

                while (reader.Read())
                {
                    Klant klant = new Klant((int)reader["klantid"], (string)reader["naam"], (string)reader["adres"], (string)reader["telefoonnummer"], (string)reader["woonplaats"], (string)reader["email"], (string)reader["wachtwoord"], (int)reader["klantpasid"]);
                    result.Add(klant);
                }
                reader.Close();
            }
            return result;
        }

```

`klantpasid` is the one that also can return a null value instead of an integer. 
Here is the class where the klantpasid is in the construtor:

```
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace FontysHotel
{
    public class Klant
    {
            // instantie variabelen
            private int klantid;
            private string naam;
            private string adres;
            private string telefoonnummer;
            private string woonplaats;
            private string email;
            private string wachtwoord;
            private int? klantpasid;

            // properties
            public int Klantid
            {
                get
                {
                    return klantid;
                }
                set
                {
                    klantid = value;
                }
            }

            public string Naam
            {
                get
                {
                    return naam;
                }
                set
                {
                    naam = value;
                }
            }
            public string Adres
            {
                get
                {
                    return adres;
                }
                set
                {
                    adres = value;
                }
            }
            public string Telefoonnummer
            {
                get
                {
                return telefoonnummer;
                }
                set
                {
                telefoonnummer = value;
                }
            }
            public string Woonplaats
            {
                get
                {
                    return woonplaats;
                }
                set
                {
                    woonplaats = value;
                }
            }
            public string Email
             {

                get
                {
                    return email;
                }
                set
                {
                    email = value;
                }
            }
            public string Wachtwoord
            {
                get
                {
                    return wachtwoord;
                }
                set
                {
                    wachtwoord = value;
                }
            }

            public int? Klantpasid
            {
                get
                {
                    return klantpasid;
                }
                set
                {
                    klantpasid = value;

                }
            }
            // Constructor
            public Klant(int klantid, string naam, string adres, string telefoonnummer, string woonplaats, string email, string wachtwoord, int klantpasid)
            {
                Klantid = klantid;
                Naam = naam;
                Adres = adres;
                Telefoonnummer = telefoonnummer;
                Woonplaats = woonplaats;
                Email = email;
                Wachtwoord = wachtwoord;
                Klantpasid = klantpasid;
            }
     }
}

```

Please provide me some help, tell me where i have to place the right code so i can continue my school project. The error i am getting now is ''' The specified conversion is invalid
'''