Minimum permissions needed to use ALTER ROLE on predefined database roles

I'm writing a script that allows my application to create Logins/Users that have permissions to create users them selves. I've read in the MS SQL 2012 documentation that if the user is a member of the db_securityadmin database role that they should have permissions to use the ALTER ROLE command on that database. I've also read numerous posts that make the same assertion.

I have created a user named MyUser whose login is also MyUser. The login is a member of the securityadmin Server Role and the user is a member of db_accessadmin, db_securityadmin, db_datareader, and db_datawriter on the database in question.

When MyUser connects to the database and attempts to execute the following command, the following error is generated:

ALTER ROLE [db_accessadmin] ADD Member [NewUser]

If I add MyUser to the db_owner database role or add the login to the sysadmin server role the command executes successfully.

What am I missing here?

Thanks,

Jamie Irwin