SNMPv3 AES256

Hi,

I'm trying to configure snmptrapd to accept incoming informs using auth SHA and priv AES2565 but can't find the syntax for AES256.
I can get it working OK using AES128.

I've tried with the following syntax in my snmptrapd.conf but despite seeing the informs coming in I'm not seeing the response going back out:
createUser -e xxxxxxx informuser sha shapassword aes 256 aespassword

Weirdly I can't see a log at all for snmptrapd either.

Would anyone be able to point me in the correct direction regarding syntax please?

Cheers.

Paul.