Pretty URLs question

[Parapet]

I have a site set up using CMSMS 1.9.3, "Motu Tane". It uses pretty URLs, and I just came across an interesting problem.

I have htaccess set up to automatically redirect the user to http://www.domainname.com when they type in domainname.com (non-www).

If the user types in http://www.domainname.com/thispage.html, they arrive at
http://www.domainname.com/thispage.html

But....

If the user types in domainname.com/thispage.html (non-www), they arrive at
http://www.domainname.com/index.php?page=thispage.html.
They are redirected to the "www" and they do see the correct page, but a dynamic URL gets displayed and not the pretty URL, yet it has the pretty url extension added.

While I understand that the dynamic URL is the internal CMSMS mechanism for writing the page location, is there any way by which someone typing in the URL without the www can be directed to the pretty URL and not the dynamic one?

Thanks in advance for any insight on this problem.

P