no encryption type specified

Hi
I have created a user in snmpv3 as myuser n gave a password.
Its also feching data by snmpget.
Then I created another user as arika and the password is same as myuser..
Now I am trying to change the password by the command :

snmpusm -v 3 -u arika -l authNoPriv -a MD5 -A my_password localhost passwd new_password new_passphrase

But its giving me the error : "no encryption type specified, which I need in order to know to change the key".
What should I do? Do I need to change my snmpd.conf file?
Plz guide..
Thanx

you need to provide either of -X or -x option with required argument