How do I access a control in the HeaderTemplate of my GridView

Question

I want to have a DropDownList in the header of my GridView. In My codebehind I can't seem to access it. Here is the HeaderTemplate:

<asp:TemplateField SortExpression="EXCEPTION_TYPE">
    <HeaderTemplate>
        <asp:Label ID="TypeId" runat="server" Text="Type" ></asp:Label>
        <asp:DropDownList ID="TypeFilter" runat="server" AutoPostBack="true">
        </asp:DropDownList>
    </HeaderTemplate>
    ...
</asp:TemplateField>

And here is the section in the code behind where I am trying to access the control 'TypeFilter'.

protected void ObjectDataSource1_Selected(object sender, 
                                          ObjectDataSourceStatusEventArgs e)
{
    DataTable dt = (DataTable)e.ReturnValue;
    int NumberOfRows = dt.Rows.Count;
    TotalCount.Text = NumberOfRows.ToString();
    DataView dv = new DataView(dt);
    DataTable types = dv.ToTable(true, new string[] { "EXCEPTION_TYPE" });
    DropDownList typeFilter = (DropDownList)GridView1.FindControl("TypeFilter");
    typeFilter.DataSource = types;
    typeFilter.DataBind();

}

You will notice that I am trying to use FindControl to get a reference to the DropDownList Control. This call returns null instead of returning the control. How do I get access to the control?