GridView“网格”触发的事件pageindexchange未被处理。

public partial class WebForm2 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            string connectionString =
            WebConfigurationManager.ConnectionStrings["base"].ConnectionString;
            string selectSQL = "SELECT author,book FROM ListItem";
            SqlConnection con = new SqlConnection(connectionString);
            SqlCommand cmd = new SqlCommand(selectSQL, con);
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            adapter.Fill(ds, "ListItem");
            grid.DataSource = ds;
            grid.DataBind();  
        }
        protected void grid_PageIndexChanged(object sender, EventArgs e)
        {
            grid.PageIndex=e.NewPageIndex;//have a error    grid.DataBind();                                      

        }
    }
public partial class WebForm2 : System.Web.UI.P