release build breaks in src/release/scripts/split-file.sh

I can't successfully build a release of RELENG_5 on my build machine 
(releng_5_2, releng_5_1 and releng_4 are OK) because of an issue with
/bin/sh manifesting itself in the split-file.sh script. Here's the
problem:

$ uname -r	# this is the build machine
5.1-RELEASE-p16
$ i=1
$ i=$((i + 1))
arith: syntax error: "i + 1"

$ uname -r
5.3-ALPHA
$ i=1
$ i=$((i + 1))
$ echo $i
2

Is this a known issue and has anyone got a workaround for this (I can't
upgrade the build machine, at least not a.t.m.)

thanks,
Ruben