Wordpress Multisite Installation bug


I have a wordpress multisite installation with the following structure:
http://example.com -> Main Site
http://example.com/internal_page -> Internal Page on Main Site
http://example.com/site1 -> Internal Site
http://example.com/site2 -> Internal Site 2 , etc
My problem is that if I put "www" at the beginning of the address on internal page on the main site, it redirects to the main site home, but it doesn't happen for internal sites:
http://www.example.com/internal_page redirects to http://example.com -> Wrong redirect
http://www.example.com/site1 redirects to http://example.com/site1 -> Right redirect
I've been doing this test with two different themes with no plugin installed, and it looks like it is a WP bug.

Hope this make sense for you and hopefully you could help me.

Thanks in advance,