Given:

        $yaml = <<<YAML
array:
    - option:
        # A comment
        value: 'a value'
YAML;
        dump(\Symfony\Component\Yaml\Yaml::parse($yaml));

I get:

  [Symfony\Component\Yaml\Exception\ParseException]
  Unable to parse at line 4 (near "    value: 'a value'").

I got this error just after upgrading from Symfony 3.3.13 to 3.3.14 inside my httplug config section. I followed instructions in its documentation example in http://docs.php-http.org/en/latest/integrations/symfony-bundle.html#plugins and added a comment just between - add_host: and host: (the docs have some lines with 8 spaces as indentation, I've used both 8 and 4 just in case but made no difference). If I remove the comment it works fine for both versions 3.3.13 and 3.3.14.