problem creating database..

hey everyone, im trying to create a database on freebsd, using postgresql
7.3. when i try and create a database with createdb i get:

su-2.05b$ id
uid=1005(postgres) gid=1005(postgres) groups=1005(postgres)
su-2.05b$ pwd
/usr/local/pgsql
su-2.05b$ createdb testdb
FATAL 1: user "postgres" does not exist
psql: FATAL 1: user "postgres" does not exist
createdb: database creation failed

so i
su-2.05b$ createuser postgres
Shall the new user be allowed to create databases? (y/n) y
Shall the new user be allowed to create more new users? (y/n) y
FATAL 1: user "postgres" does not exist
psql: FATAL 1: user "postgres" does not exist
createuser: creation of user "postgres" failed

any ideas on whats wrong?
thanks a lot!