Cannot get a reference to TextBox from NumericUpDownExtender when inside another element

<fieldset style="float:left;text-align:left;width:230px;"><asp:TextBox ID="TxtNectarPoints" runat="server"  ></asp:TextBox><ajaxToolkit:NumericUpDownExtender ID="NumericUpDownExtender1" runat="server" Maximum="100" Minimum="0"  Step="5" 
TargetControlID="TxtNectarPoints" Width="50" ></ajaxToolkit:NumericUpDownExtender></fieldset>

I can get it to work once I take it out of the FieldSet element for example. In practice it is placed inside a ModalPopupExtender control.

This example is to verify whether it was inoperable inside other  Html elements besides a Ajax control.