zfs incremental backup - destination has been modified

Hi everyone,

I am trying todo an incremental backup of a zone(volume) 
tank/vol1/vserver  to another pool backup on the same machine:

creating the snapshots is working. The first "full" copy works, but the 
second one fails.
I just followed the man page. I am doing something wrong?

Greetings,

Stephan

# zfs list
NAME                       USED  AVAIL  REFER  MOUNTPOINT
backup                    3.59G  69.7G    96K  /backup
backup/vol1               3.59G  69.7G    96K  /backup/vol1
backup/vol1/vserver       3.59G  69.7G  3.59G  /backup/vol1/vserver
backup/vol1/vserver_at_diff   104K      -  3.59G  -
tank                      21.7G  2.75G    18K  /tank
tank/usr                  21.6G  2.75G  21.3G  /usr
tank/usr/ports             334M  2.75G   262M  /usr/ports
tank/usr/ports/distfiles  72.0M  2.75G  72.0M  /usr/ports/distfiles
tank/var                  93.5M  2.75G  92.5M  /var
tank/var/log              1000K  2.75G  1000K  /var/log
tank2                       225G  41.1G    96K  /tank2/
tank2/vol1                  225G  41.1G   221G  /tank2//vol1
tank2/vol1/vserver         3.62G  41.1G  3.59G  /tank2//vol1/vserver
tank2/vol1/vserver_at_init    31.1M      -  3.59G  -
tank2/vol1/vserver_at_diff    1.41M      -  3.59G  -
tank2/vol1/vserver_at_diff2    996K      -  3.59G  -

# zfs snapshot tank2/vol1/vserver_at_diff

# zfs send tank2/vol1/vserver_at_diff | zfs receive -dv backup
receiving full stream of tank2/vol1/vserver_at_diff into 
backup/vol1/vserver_at_diff
received 3.42Gb stream in 565 seconds (6.20Mb/sec)

# zfs send -i tank2/vol1/vserver_at_diff tank2/vol1/vserver_at_diff2 | zfs 
receive -vd backup
receiving incremental stream of tank2/vol1/vserver_at_diff2 into 
backup/vol1/vserver_at_diff2
cannot receive: destination has been modified since most recent snapshot
cannot send 'tank2/vol1/vserver_at_diff2': Broken pipe

# zfs snapshot tank2/vol1/vserver_at_diff2

# zfs send -i tank2/vol1/vserver_at_diff tank2/vol1/vserver_at_diff2 | zfs 
receive -vd backup
receiving incremental stream of tank2/vol1/vserver_at_diff2 into 
backup/vol1/vserver_at_diff2
cannot receive: destination has been modified since most recent snapshot
cannot send 'tank2/vol1/vserver_at_diff2': Broken pipe