Não consigo usar o FileUpload

<asp:FileUpload ID=FileUpload1 runat=server />
<asp:Button ID=Button1 runat=server Text=Button onclick=Button1_Click />

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        
    }


    protected void Button1_Click(object sender, EventArgs e)
    {
        Label1.Text = FileUpload1.PostedFile.FileName;
    }
}