The script complained about me not having gzip installed, which was wrong, so I tried to find what was going on.
Line 108 was the culprit.

Line 98-111

MKISOFS=''
for i in $PATH ; do
if [ -x "$i/mkisofs" ] ; then
MKISOFS="$i/mkisofs"
break
fi
done
GZIP=''
for i in $PATH ; do
if [ -x "$i/gzip" ] ; then
MKISOFS="$i/gzip"
break
fi
done

Just change it to the following and everything will run fine:

MKISOFS=''
for i in $PATH ; do
if [ -x "$i/mkisofs" ] ; then
MKISOFS="$i/mkisofs"
break
fi
done
GZIP=''
for i in $PATH ; do
if [ -x "$i/gzip" ] ; then
GZIP="$i/gzip"
break
fi
done