Fatal error: Call to undefined function WP Default Theme

I’m doing some troubleshooting on my site so I disabled all my plugins, and I also selected to revert back to the WP default theme:

http://ww.wp.xz.cn/extend/themes/default

However, when I selected the default theme I’m getting an error message while in both the preview mode, as well as if I publish it, I get the same header error message on the front end. I was able to revert back to my preferred theme, but even though I’m not wanting to use the default theme, I would like to know why I’m getting this error with the default theme:

Fatal error: Call to undefined function: home_url() in /home/myname/mysiteurl.com/wp-content/themes/default/header.php on line 41

Line 41 of the default template, is this code:

<h1><a href="<?php echo home_url(); ?>/"><?php bloginfo('name'); ?></a></h1>

Could it be caused by some other themes I have installed, even though they’re not activated? Or caused by what? Thanks in advance for your help. If you need to view my source code, the site is: linkandblog (dot com).