C# – how Aggregate=”sum” For TemplateColumns In RadGrid (Telerik) -> showing in footer

my TemplateColumn In RadGrid (Telerik) is like below :

                    <telerik:GridTemplateColumn FilterControlAltText="Filter TemplateColumn_Benefit"
                        HeaderText="Benefit" UniqueName="TemplateColumn_Benefit" FilterImageToolTip="Filter">
                        <ItemTemplate>
                            <asp:Label ID="lblBenefitInsideGrd" runat="server" Font-Size="11px" Text='<%# (bool)Convert.IsDBNull(Eval("Benefit")) ? "<span class=\"lblInsideGrd\">Empty</span>" : String.Format("{0:#,0 Dollar;#,0- Dollar}", Eval("Benefit")) %>'></asp:Label>
                        </ItemTemplate>
                        <HeaderStyle HorizontalAlign="Center" VerticalAlign="Middle" Width="60px" />
                        <ItemStyle HorizontalAlign="Center" VerticalAlign="Middle" Width="60px" />
                    </telerik:GridTemplateColumn>  

the showfooter property of RadGrid Is true …
when add the below properties to this template column , so we will have an exeption …

properties : FooterText="my sum" Aggregate="Sum"

error message : Sum is not supported for type "System.Object"

how can i have sum of such these columns in radgrid ?

thanks in advance