what can i do to fix error

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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void buttonIndex_Click_Click(object sender, EventArgs e)
        {


            if (openFileDialog.ShowDialog() != DialogResult.OK)
                return;

            // parse each file and build the inverted index.
            foreach (string fileName in openFileDialog.FileNames)
            {
                string text = File.ReadAllText(fileName).Replace("\r", " ").Replace("\n", " ");
                string[] terms = text.Split(" ".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);

                // parse each term (word) in text file and put it in dictionary
                foreach (string term in terms)
                {
                    if (!InvertedIndex.ContainsKey(term))
                        InvertedIndex.Add(term, new List<string>());

                    if (!InvertedIndex[term].Contains(fileName))
                        InvertedIndex[term].Add(fileName);
                }
            }


            // update label
            labelIndex.Text = openFileDialog.FileNames.Length.ToString() + " files indexed";
        }


    }
}