Format of the initialization string does not conform to specification starting at index 0.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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.Shapes;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Configuration;
using System.Net;
using System.Collections.Specialized;
using System.Data;
using System.Text.RegularExpressions;
namespace Sup
{
    /// <summary>
    /// Interaction logic for Signup.xaml
    /// </summary>
    public partial class Signup : Window
    {
        public Signup()
        {
            InitializeComponent();
        }
        private void Reset()
        {
            textBoxAddress.Text = "";
            textBoxEmail.Text = "";
            textBoxFirstName.Text = "";
            textBoxLastName.Text = "";
            passwordBox1.Password = "";
            passwordBoxConfirm.Password = "";
        }
        private void registerButton_Click(object sender, RoutedEventArgs e)
        {
            if (textBoxEmail.Text.Length == 0)
            {

                MessageBox.Show("Enter an email.");

                textBoxEmail.Focus();

            }

            else if (!Regex.IsMatch(textBoxEmail.Text, @"^[a-zA-Z][\w\.-]*[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$"))
            {

                MessageBox.Show("Enter a valid email.");

                textBoxEmail.Select(0, textBoxEmail.Text.Length);

                textBoxEmail.Focus();

            }

            else
            {

                string firstname = textBoxFirstName.Text;

                string lastname = textBoxLastName.Text;

                string email = textBoxEmail.Text;

                string password = passwordBox1.Password;

                if (passwordBox1.Password.Length == 0)
                {

                    MessageBox.Show("Enter password.");

                    passwordBox1.Focus();

                }

                else if (passwordBoxConfirm.Password.Length == 0)
                {

                    MessageBox.Show("Enter Confirm password.");

                    passwordBoxConfirm.Focus();

                }

                else if (passwordBox1.Password != passwordBoxConfirm.Password)
                {

                    MessageBox.Show("Confirm password must be same as password.");

                    passwordBoxConfirm.Focus();

                }

                else
                {

                    string address = textBoxAddress.Text;

                    SqlConnection con = new SqlConnection("Properties.Settings.Default.usersConnectionString");
                    
                        con.Open();

                        SqlCommand cmd = new SqlCommand("Insert into Registration (FirstName,LastName,Email,Password,Address) values('" + firstname + "','" + lastname + "','" + email + "','" + password + "','" + address + "')", con);

                        cmd.CommandType = CommandType.Text;

                        cmd.ExecuteNonQuery();

                        con.Close();

                        MessageBox.Show("You have Registered successfully.");

                        Reset();
                    
                }

            }
        }
    }
}