Insert problem of checklistbox value with table that hold primary key

My Code Is Not Working If I Assigned Primary Key At Cust_Id With Not Null Values ......If I Run This Code Without Primary Key Its Work And Save All Checklistbox Item In Customer Column  But its insert values without sequence 

create table Customer 
(
cust_id numeric(10) null,
cust_name nvarchar(max)  null,
cust_machine2 nvarchar(max) null,
);

<pre>using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Data.SqlClient;
using System.Text.RegularExpressions;
using System.Xml.Linq;


namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        string res;
        //get checked value from CheckBox which is located in Stack Panel…………….
        
        
        
 
        
        
        private void btngetvalue_Click(object sender, RoutedEventArgs e)
        {


            
            res = "";
            foreach (UIElement ul in spforcheckbox.Children)
            {
               

                if (ul is CheckBox)
                {
                    CheckBox chk = (CheckBox)ul;
                    if (chk.IsChecked == true)
                    {
                       

                        string dbconnection = (@"Data Source=EMIII-PC; Initial Catalog=gymdatabase;Integrated Security=True");
                        SqlConnection conn = new SqlConnection(dbconnection);
                        {

                            conn.Open();
                            using (SqlCommand cmd = new SqlCommand("INSERT INTO Customer(cust_id,cust_name,cust_machine2) VALUES  ( @custid,@custname,@custmach2 )", conn))
                            {

                                cmd.Parameters.AddWithValue("@custid", txtcid.Text);
                                cmd.Parameters.AddWithValue("@custname", txtcname.Text);
                                cmd.Parameters.AddWithValue("@custmach2", chk.Content);
                                int rows = cmd.ExecuteNonQuery();
                                
                                                         


                            }
                        }
                    }
                }
            }
        
        }

        private void txtcid_TextChanged(object sender, TextChangedEventArgs e)
        {

        }

        private void txtcname_TextChanged(object sender, TextChangedEventArgs e)
        {

        }
    }
}

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>

        <Grid.RowDefinitions>
            <RowDefinition Height="35"></RowDefinition>
            <RowDefinition Height="35"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="35"></RowDefinition>
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100"></ColumnDefinition>
            <ColumnDefinition Width="*"></ColumnDefinition>
        </Grid.ColumnDefinitions>

        <TextBlock Grid.Row="0" Grid.ColumnSpan="2" Text="CheckBox in W P F" TextAlignment="Center" FontSize="20"  Foreground="Red"  Padding="3,0,25,35"></TextBlock>

        <Label x:Name="textcid" Grid.Column="0" Content="Cust Id" Padding="30,10,10,3" Margin="0,30,0,5" Grid.RowSpan="2"/>
        <Label x:Name="textcname" Grid.Row="2" Grid.Column="0" Content="Cust Name" Padding="30,10,10,3"></Label>
        <TextBox x:Name="txtcid" Grid.Row="1" Grid.Column="1"  Margin="136,0,131,6" Width="150" TextChanged="txtcid_TextChanged"></TextBox>

        <StackPanel Grid.Row="2" x:Name="spforcheckbox" Grid.Column="1" Width="150" Height="125" removed="Azure" Margin="134,56,133,34" >

            <CheckBox Margin="4" Content="Chest Press" IsChecked="True"  />
            <CheckBox Margin="4" Content="Leg Press" />
            <CheckBox Margin="4" Content="Butterfly" />
            <CheckBox Margin="4" Content="Sitted ROwing" />

        </StackPanel>

        <Button Grid.Row="3" Grid.Column="0" Content="Submit" x:Name="btngetvalue" Margin="5,5,5,5" Click="btngetvalue_Click" ></Button>

        <Label Grid.Row="3" Grid.Column="1" x:Name="lblmessage" Margin="5,5,5,5" ></Label>
        <TextBox x:Name="txtcname" Grid.Row="2" Grid.Column="1"  Margin="134,7,133,177" Width="150" TextChanged="txtcname_TextChanged"/>

    </Grid>
</Window>