Use of uninitialized value in modulus at /usr/share/fai/setup-storage//Sizes.pm line 599

my problem is quite similar to https://bugs.launchpad.net/ubuntu/+source/fai/+bug/941959, but my permissions on /dev/sda (per that case's solution) are correct. still about 1 out of every 20 builds will succeed for me. most of the time, my builds bomb out with the following messages:

1) The release of Ubuntu:
  Description: Ubuntu Precise
  Release: 12.04
  Codename: precise
2) The version of the packages:
ii fai-client 3.4.8ubuntu2 Fully Automatic Installation client package
ii fai-doc 3.4.8ubuntu2 Documentation for FAI
ii fai-server 3.4.8ubuntu2 Fully Automatic Installation server package
ii fai-setup-storage 3.4.8ubuntu2 automatically prepare storage devices
3) What you expected to happen:
  fai should format my disk according to the configuration i supply.
4) What happened instead:
  setup-storage crashes with the error message:

Starting setup-storage 1.3
Using config file: /var/lib/fai/config/disk_config/FAIBASE
Can't exec "mdadm-startall": No such file or directory at /usr/sbin/setup-storage line 138, <$config_file> line 1.
    Creating directory "/var/lock/lvm"
    Finding all volume groups
  No volume groups found
Executing: mdadm --examine --scan --verbose -c partitions
Use of uninitialized value in modulus (%) at /usr/share/fai/setup-storage//Sizes.pm line 599.
Use of uninitialized value $block_size in modulus (%) at /usr/share/fai/setup-storage//Sizes.pm line 599.
Illegal modulus zero at /usr/share/fai/setup-storage//Sizes.pm line 599.
Error in task partition. Traceback: task_error task_partition task task_install task task_action task main
FATAL ERROR. Installation stopped.

my config:

disk_config /dev/sda disklabel:msdos fstabkey:uuid

primary /boot 1024 ext4 rw,noatime,errors=remount-ro
primary swap 1024 swap rw
primary / 0- ext4 rw,noatime,errors=remount-ro

I'm banging my head against my desk here. any ideas?