Server side validation of Fileupload control not working

   

I've tried this method in my code behind to try and validate that the Fileupload control has a file or not. I get the error message the the event "Server Validate" cannot be found. Can anyone tell why? Thanks.
Public Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs) Handles CustomValidator1.ServerValidate
       
' Get file name

        Dim uploadAvatar As FileUpload = DirectCast(AdvertisementForm.FindControl("uploadAvatar"), FileUpload)

        Dim UploadFileName As String = uploadAvatar.PostedFile.FileName

        If UploadFileName = &quo ...