is this a bug?

[naruto canada]

function fact {
  local n=$1
  if [ "$n" -eq 0 ]; then
    return 1
  else
    fact $(( $n - 1 ))
    return $(( $n * $? ))
  fi
}

for i in `seq 0 11`; do
fact $i ; echo $?
done


1
1
2
6
24
120
208
176
128
128
0
0

the results are wrong for 6 and above.
is this a bug?