Create command_rule with ALTER USER

Please help

I want to create command_rule to prevent other user to alter user on GARRY as below:

begin DVSYS.DBMS_MACADM.CREATE_COMMAND_RULE(command=> 'ALTER USER', rule_set_name => 'Can Maintain Own Account', object_owner => DBMS_ASSERT.SCHEMA_NAME('GARRY'), object_name => '%',enabled => 'Y'); end;

/

And I got this error message:

ERROR at line 1:

ORA-28104: input value for object_owner is not valid

ORA-06512: at "DVSYS.DBMS_MACADM", line 1842

ORA-06512: at line 1"

Is there any thing wrong with my command?

Thanks