bilbo removes all   codes from the HTML Editor Tab when I switch to the Visual Editor Tab

I've been using biblo for updating my blogs at wordpress.com, recently. Since I blog about programming, I usually need to depict some code snippets on each post. For clarity and readability, code snippets that I put on my blog should be indented using space characters. But the problem is that when I put   characters (non-breaking space) in the html code of my posts, bilbo automatically removes them. This happens when I switch from the HTML Editor tab to the Visual Editor tab.