Error in C# random addition program

I keep getting an error on the line that states int userIN = int.Parse(answerBox.Text); I don't see why this is happening. I'm sure it just something I'm overlooking, but I've been sitting here totally baffled.

An unhandled exception of type 'System.FormatException' occurred in mscorlib.dll

Additional information: Input string was not in a correct format.

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

        private void Form1_Load(object sender, EventArgs e)
        {
            // First Random Nummber
            int rand1;
            Random rn1 = new Random();
            rand1 = rn1.Next(500) + 100;
            number1.Text = rand1.ToString();
            //Second Random Number
            int rand2;
            rand2 = rn1.Next(500) + 100;
            number2.Text = rand2.ToString();
            // Answer
            int anw = rand1 + rand2;
            int answ = rand1 + rand2;
            // Check
            int userIN = int.Parse(answerBox.Text);
            if (answ == userIN)
            {
                feedback.Text = "Correct";
            }
            else
            {
                feedback.Text = "incorrect";
            }
        }
    }
}