Vanity Url ID issue

Vanity Url ID issue

I am trying to do vanity url.
I want this http://mydomain.com/series/profile.php?profile_id=751130015 to become something like this
http://mydomain.com/series/751130015
I've tried creating a new file '.htaccess' on the directory and add this code:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /profile.php?profile_id=$1
Yet, I am still getting an error when I try to acccess
http://mydomain.com/series/751130015
Am I missing anything?
The error I am getting is:
Object not found!
The requested URL was not found on this server. If you entered the URL manually please check your spelling and try again.
If you think this is a server error, please contact the webmaster.
Error 404