Replace iframe in getData()

I need to replace an iframe with something simple for example, 'b' tag, I am doing:

dataProcessor.htmlFilter.addRules({
   elements: {
      iframe: function(element) {
         return new CKEDITOR.htmlParser.fragment.fromHtml('<b>abc</b>');
      }
   }
});

But it returns an empty string when doing editor.getData(). What might be the problem here?