301 redirect is working for www.example.com but not for ....

Question

I am getting a strange problem with 301 redirect it the code is working for with www in the link and not working for with out www

RewriteBase /
RewriteRule ^api$ http://example.com/topic/api [R=301,L]

if i type http://www.example.com/api it works but if i do http://example.com/api it doesnt work

Adv thanks prasanth