can't fsck_ext2fs on a non sliced disk on 5.1

[Marc Hufschmitt]

I got this message :

# fsck_ext2fs -d /dev/ad3
** /dev/ad3
BAD SUPER BLOCK: MAGIC NUMBER WRONG
ioctl (GCINFO): Inappropriate ioctl for device
/dev/ad3: can't read disk label

is this due to geom or to FreeBSD 5.1? It worked on a 4.6.2.
Though /dev/ad3 could be mounted when it was clean.