got "invalid argument" error when using dd

[Ji]

Hi all,
I am using FreeBSD-7.2-release-amd64. I got an got "invalid argument" error
when using dd

dd if=s1.image of=/dev/ad4s1 bs=1m
dd: /dev/ad4s1: Invalid argument
12284+1 records in
12284+0 records out
12880707584 bytes transferred in 746.571300 secs (17253151 bytes/sec)

echo $?
1

(s1.image is the image file of /dev/ad4s1 generated by "dd if=/dev/ad4s1
of=s1.image bs=1m"  ahead. )
The error occurs at the moment that dd almost finishes.
I don't quite understand the error, Can anybody enlighten me? Thanks.

Ji