fails to mount md /tmp

i get

    # /sbin/mdmfs -s 64m -M md /tmp
    mdmfs: mdconfig (attach) exited with error code 1

kernel has /dev/md

    # grep md /sys/i386/conf/RAN
    #device         amd             # AMD 53C974 (Tekram DC-390(T))
    device          md              # Memory "disks"

but their ain't no /dev/md0

    # ls /dev/md*
    /dev/mdctl

i do have /etc/rc.conf set up the same as on systems which
mount it successfully

    # grep tmp /etc/rc.conf
    tmpmfs="YES"            # Set to YES to always create an mfs /tmp, NO to never
    tmpsize="64m"           # Size of mfs /tmp if created
    clear_tmp_enable="YES"  # Clear /tmp at startup.

randy