Segmentation fault

[seba]

GNU bash, version 3.2.25(1)-release (i686-pc-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.

#!/bin/sh

fib() {
       n=$1
       [ $n == 0 -o $n == 1 ] && return $n
       fib $(($n-1))
       ret1=$?
       fib $(($n-2))
       ret2=$?
       return $(($ret1 + $ret2))
}

for ((i=0;$i<36;i++))
do
       fib $i
       echo "n=$i=>$?"
done