www.subdomain.domain.com doesn't work.

[ziggy]

I am having some problems with DNS records.

I have a subdomain that I can access by typing:
http://subdomain.domain.com

If I want to access it by

http://www.subdomain.domain.com there is an error.

I have tried to solve this problem by adding this dns record:

Host: www.subdomain.domain.com (http://www.subdomain.domain.com)
Type: CNAME
Target: domain.com

I still get an error. :(

How do I fix this problem?