Tokens vanishing in mustache.php

test.php

<?php

require 'src/Mustache/Autoloader.php';
Mustache_Autoloader::register();

$m = new Mustache_Engine([
    'partials' => [
    ],
]);

$tpl = '<h1>{{{blah}}</h1>';

echo $m->render($tpl, [
    'blah' => 'hello world!'
]);

// => <h1>hello world!/h1>