Unable to find Control ID of Dropdownlist Control of a UserControl from a ContentPage

Hi,
I am implementing Master Pages in my existing project.

I am getting following Error:

" Unable to find control id '_ctl0:ContentPlaceH?older1:drop1:drop1' referenced by the 'ControlToValidate' property of 'valCust1'. "

I have a ContentPage which is refering to a UserContol. This UserControl has a DropDownList Control which i need to validate.

I am using a CustomValidator to validate the Dropdownlist and this CustomValidator is placed in the ContentPage.

The ControlToValidate Property of the CustomControl is assigned programatically as
(in ASPX.cs)

Control CTRL1;

ContentPlaceHolder cntplcHolder = (ContentPlaceHolder)?this.Master.FindCont?rol("ContentPlaceHol?der1");


 CTRL1 = cntplcHolder.FindCon?trol("drop1").FindCo?ntrol("drop1");

 valCus ...