set blank password for PostgreSQL user

Question

I use postgresql command 'createuser myusername'. It prompts me for password always. If I leave password blank it gives an error that no password is specified.

I simply want to create a user with blank/no password and I'm not able to do it with Postgresql 9 under mac os x.

What steps should I take to create a user with no password. The command always asks me to enter a password.