gpart: can not add MBR partitions with "gpart add -t mbr"

Having a 1TB disk with a MBR partition layout created successfully with
"gpart create -s MBR ada4", which is shown as

Geom name: ada4
modified: false
state: OK
fwheads: 16
fwsectors: 63
last: 1953525167
first: 63
entries: 4
scheme: MBR
Consumers:
1. Name: ada4
   Mediasize: 1000204886016 (931G)
   Sectorsize: 512
   Mode: r0w0e0


it is impossible for me to create a partition with 

gpart add -t mbr ada4

as it can be read in the manpage of gpart. Whatever I do, I receive a
useless error from gpart

gpart: Invalid argument

with no further information. 

In kernel, I already added GEOM_MBR. geom_part_mbr and geom_part_ldm
kernel modules are then already loaded.

I tried to figure out what's going wrong but this seems highly
enigmatic. 

What am I doing wrong?

Regards and thanks,

Oliver