upgraded to 4.3 but having probs adding a plugin

I just upgraded to 4.3 and added the image2 plugin to the plugins folder. I added this..

CKEDITOR.replace( 'editor1', {
   //extraPlugins: 'image2',
   height: 250,
   width: 650

which works when I comment out the extraPlugins line but fails otherwise. In IE 11 debugger, it stops at line

SCRIPT5007: Unable to get property 'icons' of undefined or null reference
File: ckeditor.js, Line: 220, Column: 249