Undo fails in IE when called inside a paragraph

how to reproduce:

• open an empty document
• insert text
• set formatting to paragraph
• type some more text
• hit ctrl + Z to undo

an error would occur on internet explorer saying that an access to an unexpected property has been made.

There is an issue in the DefaultHistory.getNode() on node.getChildNodes.getLength() (line 258) when node is a text node.

The issue has occurred on IE7, haven't tested on IE6.