psql: FATAL: user "root" does not exist createdb: database creation failed

Hi to all I'm Jeoffrey, and I'm a newbie to postgreSQL. My problem is
that when i execute the command createdb <database name> i always get a

[root(at)webdata root]# createdb ibmadb
psql: FATAL: user "root" does not exist
createdb: database creation failed

Then I tried to create the user root by the command:

createuser <username> and i always get this message also:

[root(at)webdata root]# createuser
Enter name of user to add: root
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) n
psql: FATAL: user "root" does not exist
createuser: creation of user "root" failed

my postgresql version is 7.3.4 and my OS is FEDORA CORE 1 x86_64 bit
architecture.

What is wrong about it? please help.

Thanks,

Jeoffrey