Index: wp-includes/js/tinymce/plugins/wpeditimage/editor_plugin_src.js
===================================================================
--- wp-includes/js/tinymce/plugins/wpeditimage/editor_plugin_src.js	(revision 25742)
+++ wp-includes/js/tinymce/plugins/wpeditimage/editor_plugin_src.js	(working copy)
@@ -160,7 +160,9 @@
 				if ( cls == 'aligncenter' )
 					div_cls += ' mceIEcenter';
 
-				return '<div class="'+div_cls+'"><dl id="'+id+'" class="wp-caption '+cls+'" style="width: '+( 10 + parseInt(w) )+
+				w = parseInt( w ) + 10;
+
+				return '<div class="'+div_cls+'"><dl id="'+id+'" class="wp-caption '+cls+'" style="width: '+w+
 				'px"><dt class="wp-caption-dt">'+img+'</dt><dd class="wp-caption-dd">'+cap+'</dd></dl></div>';
 			});
 		},