.htaccess 301 redirect index.php, index.html in subdomain

Hi,
I have this page for a client:
http://irrevocable-trust.ultratrust.com/

But when I use:
http://irrevocable-trust.ultratrust.com/index.php
http://irrevocable-trust.ultratrust.com/index.html

the page isn't found. I wish to use .htaccess to do a 301 redirect from
http://irrevocable-trust.ultratrust.com/index.php
to
http://irrevocable-trust.ultratrust.com/

I tried this:
RewriteRule ^index.php$ http://irrevocable-trust.ultratrust.com/ [R=301, L]

But it doesn't work.
I have this in the same .htaccess file for the subdomain like:
RewriteRule ^intentionally-defective-grantor-trust.html$ http://irrevocable-trust.ultratrust.com/irrevocable-trust-agreement.html [r=302]

and it works. So I don't know why this doesn't work:
RewriteRule ^index.php$ http://irrevocable-trust.ultratrust.com/ [R=301, L]

Thank you,
Victor