[C#]System.InvalidCastException

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Geo_Rechner
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnRechnen_Click(object sender, EventArgs e)
        {
            double dLaengePosition1 = 0;
            double dBreitePosition1 = 0;
            double dLaengePosition2 = 0;
            double dBreitePosition2 = 0;
            double dDist = 0;

            dLaengePosition1 = Convert.ToDouble(tBoxP1L);
            dBreitePosition1 = Convert.ToDouble(tBoxP1B);
            dLaengePosition2 = Convert.ToDouble(tBoxP2L);
            dBreitePosition2 = Convert.ToDouble(tBoxP2B);

            dDist = 6378.388 * Math.Acos(Math.Sin(dBreitePosition1) * Math.Sin(dBreitePosition2) + Math.Cos(dBreitePosition1) * Math.Cos(dBreitePosition2) * Math.Cos(dLaengePosition2 - dLaengePosition1));

            lbAusgabe.Text += Convert.ToString(dDist);
            lbAusgabe.Text += " Kilometer.";
        }
    }
}