zfs change passphrase

I know ZFS isn't a proxmox specific feature, but the version used in proxmox is different from the one documented in Oracle.

Anyway, this irrelevant point apart, when I try zfs change-key, I get this error:
Code:
# zfs change-key -i poolz/secdata
Key change error: Parent must be encrypted.

If I don't mount it using zfs mount -al it doesn't mount, so I think it must be encrypted...