Issue with image insertion & TinyMCE/Text editor

I’ve picked up an issue with images being replaced when switching between TinyMCE and Text editor. Here are the steps:

1. Insert html into text editor. e.g.

<img class="alignnone size-full wp-image-21710" src="https://en.support.files.wordpress.com/2009/02/appearance_menu1.png" alt="" width="154" height="231" />

2. Switch to visual editor. Image isn’t displayed – get spinner graphic instead.

3. switch back to text editor and html src, width and height is replaced with “undefined” e.g.

<img class="alignnone size-full wp-image-21710" src="undefined" width="undefined" height="undefined" alt />

Tested on WordPress local installation in 3.5-beta-1 and 3.5-beta1-22104, and on live site (make.wp.org/support)