Can't change Column name for grid view on asp.net

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ASP.NET

Gridview

Hi, im making a website which allows a user to select a film from a list of genres and then save that film choice to a database. I've also added a grid view control which allows the user to view the most selected films. Now i've got that grid view control to work as it does display the most selected films, however the column name for the column with the number of times a film has been selected has the default name of Column 1. I added a bit of code to change the name of this to "Number" but it isnt allowing me to do this? It displays an error saying "System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index"


Heres my code I've used, i've marked the place in the code where the error is located.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Data;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void ButtonDatabase_Click(object sender, EventArgs e)
    {
        // set up data connection
        SqlConnection cs = new SqlConnection("Data Source=MASTER\\MASTER;Initial Catalog=FilmDB;Integrated Security=True");
        //set up adapter manager 
        SqlDataAdapter da = new SqlDataAdapter();
       
        DataSet ds = new DataSet();
        da.SelectCommand = new SqlCommand("Select FilmName, COUNT(*) FROM Film Group BY FilmName Having COUNT(*) > 1", cs);
        
        
        da.Fill(ds, "Film");
        gvFilms.DataSource = ds;
        gvFilms.DataBind();
 //This is where the error is       gvFilms.Columns[0].HeaderText = "Number";
        btnDatabase.Enabled = false;

        
    }
   }