Why is this code not allowing me to add a checkbox to it?

<asp:repeater ID="RptTrip" Runat="server" EnableViewState="false">
   <ItemTemplate>
       <asp:Checkbox AutoPostBack="True" ID="cb" runat="server"
Checked="false"></asp:Checkbox>
       <trip_detail_info trip_detail_id ='<%#
Container.DataItem("trip_detail_id")%>'
          leg_start_dt =    '<%# Container.DataItem("leg_start_dt")%>'
          pickup_iso_dt = '<%# Container.DataItem("pickup_iso_dt")
%>' />
   </ItemTemplate>
</asp:repeater>

I keep getting a "Control 'RptTrip__ctl0_cb' of type 'CheckBox' must
be placed inside a form tag with runat=server. " message even though I
obviously already have that tag in the control.