Hi guys!

Code to reproduce:

use Symfony\Component\PropertyAccess\PropertyAccess;

$data = new stdClass();
$data->pizzas = array(
    'mushroom'
);

$accessor = PropertyAccess::createPropertyAccessor();
$val = $accessor->getValue($data, 'pizzas[1]');
dump($data);

This will print:

{
  +"pizzas": array:2 [
    0 => "mushroom"
    1 => null
  ]
}

Notice the 1 key has actually been added to the underlying data. The problem seems to only happen when you're working on an array inside of an object (i.e. I could not reproduce this by working with an array at the top-level).