createuser www-data ????

I am working on a server that is using the Debian packages PHP3,
postgresql and Apache.

I believe that the Debian default is to run Apache as user www-data.

To make a connection to postgresql for PHP3 I have to create a user
www-data.

However when I attempt to createuser www-data the error message is:

invalid characters in username  www-data

Anyone suggest a workaround

Thanks

Craig