Blank Archives Pages

My brother’s blog is set up using the year/month permalinks like so:

/%year%/%monthnum%/%postname%/

He also has an archives page here:

http://nyarlo.net/archives/

If you go to a page’s permalink, everything works fine:

http://nyarlo.net/2008/05/two-thousand-and-what/

But if you go to a year or month archive page, it’s just blank:

http://nyarlo.net/2008/

Any idea what I can do to fix this so his monthly archives display correctly?