issues with geom and adding disks

[Dave]

Hello,
    I'm following the handbook section 18.3.2.1 to add a disk to a FreeBSD
6.2 system. I've got the disk plugged in and it shows up as /dev/ad6 so i
did:

dd if=/dev/zero of=/dev/ad6 bs=1k count=1

That worked fine. My problem came with fdisk. I used:

fdisk -BI ad6

and i got the message:

Invalid partition table
geom not found.

If i do:

fdisk ad6

i get a summary of the current disk partitions on ad6, all empty as this is
a new disk.
Thanks.
Dave.