Nothing is displayed in my Footer

<FooterTemplate>
<asp:DropDownList ID="ddlMaterial_Type" runat="server" Width="100%">
<asp:ListItem Value="Select"><-Select-></asp:ListItem>
</asp:DropDownList>
</FooterTemplate>

protected void GV_RowDataBound(object sender, GridViewRowEventArgs e)
   {
       if (e.Row.RowType == DataControlRowType.Footer)
       {
           DataSet ds = new DataSet();
           ds = ViewState["ds"] as DataSet;
           DropDownList ddlMaterial_Type = (DropDownList)e.Row.FindControl("ddlMaterial_Type");
           ddlMaterial_Type.DataSource = ds;
           ddlMaterial_Type.DataTextField = "DED_Name";
           ddlMaterial_Type.DataValueField = "DED_Id";
           ddlMaterial_Type.DataBind();
       }
   }