npf woes

[Edgar Pettijohn]

I'm attempting to test npf, but can't get it started. I am getting the 
following:

laptop$ sudo service npf start
Enabling NPF.
npfctl: cannot open '/dev/npf': Device not configured
npfctl: cannot open '/dev/npf': Device not configured
laptop$ ls -l /dev | grep npf
crw-------  1 root   wheel     198,       0 May 19 05:17 npf
laptop$ sudo service npfd start
Starting npfd.
npfd: cannot open '/dev/npf': Device not configured

With a simple config:

laptop$ cat /etc/npf.conf
$ext_if = { inet4(iwn0) }

alg "icmp"

procedure "log" {
                log: npflog0
}

group "external" on $ext_if {
                pass stateful out final all
}

group default {
                pass final all
                block all apply "log"
}

Have I missed a preliminary step?

Thanks,

Edgar