adding a customvalidator to page on a callback;

 Hi.
 I am creating a CustomValidator control on a callback in a webcontrol event. A ValidationSummary control resides on the Page.  In order to display a dynamic message, I create a CustomValidator control and add it to the page.  Unfortunately, the ValidationSummary on the Page is not picking it up.  Where am I going wrong?  Thanks

 
Code in webcontrol:
        CustomValidator CustomValidatorCtrl = new CustomValidator();
        CustomValidatorCtrl.IsValid = false;
        CustomValidatorCtrl.ErrorMessage = "Begin Date is duplicated.";
        this.Page.Controls.Add(CustomValidatorCtrl); 

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