Nginx throws error with `iconv('UTF-8', 'ASCII//TRANSLIT', ...)` in PHP 7.1

[mks-colibri]

Code that generates the error:

<?php
ini_set('display_errors', 1);
error_reporting(E_ALL);
iconv('UTF-8', 'ASCII//TRANSLIT', "...");

See these for reference:
docker-library/php#240
https://gist.github.com/guillemcanal/be3db96d3caa315b4e2b8259cab7d07e