DoudouLinux CD

USB device has 2 fat-formatted partitions. Boot from LiveCD.
Error description is below.

sudo persistence-gui --expert
T.Sonic 840 [4.1 Gb, usb, 2 ????? ????(?)]
/dev/sda2 vfat (3.3 Gb)
/dev/sda1 vfat (700 Mb) [DEBIAN_LIVE]
Samsung SP0411N [40 Gb, ide, 2 ??????? ????(?)]
/dev/sda3 ext3 (38 Gb)
/dev/sda2 swap (360 Mb)
/dev/sda1 vfat (730 Mb)
PC Floppy Drive [0 b, platform, 0 ??????? ??(?)]

Selected disk: T.Sonic 840 [4.1 Gb, usb, 2 ????? ??(?)]
Selected partition: /dev/sda2 vfat (3.3 Gb) [3.3 Gb]
Free space: 2.2 Gb
Selected mode: mode_root_file
Selected size: 2047 Mb
dd if=/dev/null of=/mnt/sda2/live-rw bs=2G seek=1
mkfs.ext2 -F /mnt/sda2/live-rw
Traceback (most recent call last):
File "/usr/bin/persistence-gui", line 1135, in <module>
GUI.select(ExpertMode)
File "/usr/bin/persistence-gui", line 984, in select
SelectedPart.set_persistence_file(SelectedMode, SelectedSize, reset_file=True)
File "/usr/bin/persistence-gui", line 478, in set_persistence_file
Str += commands.getoutput("dd if=/dev/null of=%s bs=%s seek=1" % (FilePath, FileSize))
TypeError: can only concatenate tuple (not "str") to tuple