First you have your Web control in the ASPX file (I hope you added it inside.
otherwise this is the cause...) and then declared in the codebehind (aspx.cs or
aspx.vb) as protected System.Web.UI.blah.blah...

If ID in ASPX file is the same as object  declaration name in the codebehind you
shouldn't get the exception. Unless you called Clear() of the container
containing your control.

Does this sound understandable?