Microsoft JScript runtime error: (...)' is null or not an object

I get this error in a simple JS - that I'm using to set focus.  THe JS is a page that is hosted in a Masterpage... What is wrong?   Why can't it find this object ?

I look at the source and it's there...

   document.getElementById(<%= cmbPREFIX.ClientID %>).focus()





  function alertSaved() {
        var oWnd = radalert("Constituent Information Added!",300,100, "TRU");
        oWnd.add_close(function () {

// ERROR HERE
                     document.getElementById(<%= cmbPREFIX.ClientID %>).focus()
   

           /// Doesn't work either -   $get('cmbPREFIX').focus();
        });