custom validator in user control

Hi: I have a simple user control with a dropdownlist, a textbox and a customvalidator. The usercontrol is on a page with a masterpage, and there are several of these usercontrols on the page, so I'm trying to compartmentalize the validation logic by putting the validator in the usercontrol, and using the clientID to get the ID of the dropdownlist:
<asp:DropDownList runat="server" ID="ddlParam" AppendDataBoundItems="true" DataTextField="paramVal" DataValueField="paramID"> <asp:ListItem Value="0" Text="Select or enter new" /> </asp:DropDownList> <asp:TextBox runat="server" ID="txtParam" MaxLength="50" /> <asp:CustomValidator runat="server" ID="cvDDL" OnServerValidate="cvDDL_ServerValidate" ClientValidationFunction="checkDD ...