error: line 9: * 12: syntax error: operand expected

I've been getting the error: line 9: * 12: syntax error: operand expected (error token is "* 12") (I entered 3, 4, 1 as a test on the script) the result that I need still shows at the end but after this error line. Why is that line showing if everything "works"?

Here's my code so you can understand what I did:

y='1'
z='0'

while [ $z = 0 ]
do
read x
        if [[ "$x" -ge "0" ]];
        then
        y=$[$x*$y]
        fi
done

echo -n "The multiplication of positive numbers entered is: $y"

exit