Php 7 "cannot use string offset as an array" - php

Question

We've recently upgraded to Php 7.4 and are encountering a potential fatal error alert for the following code block:
$total = 0;
$balance = 0;
if (is_array($company['history']) && ($histCount = count($company['history']))) {
for ($i = $histCount - 1; $i >= 0; $i--) {
$total += $company['history'][$i]['amt_total'];
$balance += $company['history'][$i]['due'];
$company['history'][ $i ]['balance'] = $balance; // EA flags as potential fatal error
$company['history'][ $i ]['total'] = $total; // EA flags as potential fatal error
}
}
Each history element in the $company has (among others) amt_total, due, balance, and total initialized, with the last two being initialized to 0.
EA flags the two lines indicated, saying "Could provoke a PHP Fatal error (cannot use string offset as an array)."
Grateful to anyone who can help me understand what's wrong here.
ADDITIONAL INFO
Someone asked where the code for defining $company comes from...
foreach ($this->orderList as $order) {
if ($order['amt_paid'] !== $order['real_amt_paid'] && !empty($order['real_amt_paid'])) {
$order['amt_paid'] = $order['real_amt_paid'];
}
$row = [];
foreach ($this->customerHistoryColumns as $col) {
$order[$col[0]] = ($order[$col[0]] === null ? "" : str_replace("&", "&", $order[$col[0]]));
switch ($col[0]) {
...
case "due":
$row['due'] = $order['amt_total'] - $order['amt_paid'];
break;
...
default:
$row[$col[0]] = $order[$col[0]];
break;
}
}
$row['balance'] = 0;
$row['total'] = 0;
$arr['history'][] = $row;
}
return $arr['history'];
The company itself is defined elsewhere; this is simply retrieving the history element.
FWIW, the code has worked flawlessly for years; it's the upgrade to Php7 that has raised this issue.