PhpStorm IDE 2017.2没有突出显示PHP 7.1.x中错误的“[]运算符不支持字符串”

I installed PhpStorm version 2017.2 and configured the PHP 7.1 under Settings > Languages and Frameworks > PHP > PHP Language Level

Unfortunately it does not highlight possible errors when using the empty array index operator on string variables ( description: http://php.net/manual/de/migration71.incompatible.php#migration71.incompatible.empty-string-index-operator )

Code:

<?php
$test = 'a string';

$test[] = 'an array value';

echo "it works";

When executing the code with PHP 7.1.x the following error occurs

Fatal error: Uncaught Error: [] operator not supported for strings in

so the code is definitely not working.

Can anyone help me with the PhpStorm configuration?