Empty paragraphs are lost after redo operation in IE6/7

Here are the steps to reproduce:

• edit a page with the new WYSIWYG editor
• write some text
• press Enter many times (at least 3 times)
• write some text again
• at this point you have two lines of text separated by some empty paragraphs
• click undo for the last text you wrote
• click redo to have it back
• the empty paragraphs between the two lines of text are lost