Variable as bash array index?

#!/bin/bash

set -x

array_counter=0
array_value=1

array=(0 0 0)

for number in ${array[@]}
do
    array[$array_counter]="$array_value"
    array_counter=$(($array_counter + 1))
done

When running above script I get the following debug output:

+ array_counter=0
+ array_value=1
+ array=(0 0 0)
+ for number in '${array[@]}'
+ array[$array_counter]=1
+ array_counter=1
+ for number in '${array[@]}'
+ array[$array_counter]=1
+ array_counter=2
+ for number in '${array[@]}'
+ array[$array_counter]=1
+ array_counter=3

Why does the variable $array_counter not expand when used as index in array[]?