Gluster users - Apr 2017 - Special Config or not Possible (5 bricks)

Hi,

i have 5 DIsks (3 3 TB, 2 3 TB)
Actual running Raid 5 on the 3 3 TB Disks and Raid 1 on the  3 TB DIsks
(dont diskuss about Raid 5 .. i know it :)
Plan was to use gluster with sharding enabled replica 3 on all 5 disks 
so parts of the File are at minimum of 3 disks (localy make 5 bricks ...)

i have tried it with loop files (5 pieces) but this configuration seems 
to not work. I can enable 4 devices but not the 5th.(in my Test only 
replica count 2 ... final plan is replica 3 ...)


Config:
root at ICE-NAS:~# gluster volume info vol1

Volume Name: vol1
Type: Distributed-Replicate
Volume ID: 16f67225-4949-4510-85c2-0486aa089ec3
Status: Started
Number of Bricks: 2 x 2 = 4
Transport-type: tcp
Bricks:
Brick1: ice-nas:/gltest/br1/ds
Brick2: ice-nas:/gltest/br2/ds
Brick3: ice-nas:/gltest/br3/ds
Brick4: ice-nas:/gltest/br4/ds
Options Reconfigured:
features.shard-block-size: 128MB
features.shard: on
transport.address-family: inet
performance.readdir-ahead: on
nfs.disable: on

Command:
root at ICE-NAS:~# gluster volume add-brick vol1 icenas:/gltest/br5/ds
volume add-brick: failed: Incorrect number of bricks supplied 1 with count 2

When i understand the Manual right i can also add a single Brick with 
sharding enabled !?
Where is my mistake ...

Greets from Germany
Holger