smartmontools question

[FreeBSD Daemon]

dear list,

i just installed smartmonutils from ports on a 4.11-release box.

when i run

# /usr/local/sbin/smartctl -a ad0

I get the error message

smartctl version 5.30 Copyright (C) 2002-4 Bruce Allen
[snip]
Smartctl open device: ad0 failed: Inappropriate ioctl for device

the same with the second disk in the system (ad2).
the same if i user /dev/ad[02]+.

what might be the problem?

TIA