can't get to main blog pages with %postname% permalink on sub-directory install

i've installed the multisite with the sub-directory configuration.

now, i created some pages as "about", and others on the main blog, with the permalink structure as "%postname%"

the problem is that now, when i want to go to the about page on the main blog, the system "thinks" that i want to look for a sub-blug that call "about", and because it isn't existed, it sends me right a way to the home page.

how do i know? because when i've changed the permalink to the normal structure (as "?page_id=2") it worked.

some one knows how to make the pages work with the %postname% permalink, and to make the system to go to the page instead of looking for the blog with the page's name?