Bare bones example of broken {{ render() }}

composer.json

{
    "require": {
        "silex/silex": "1.0.*@dev",
        "twig/twig": ">=1.8,<2.0-dev",
        "symfony/twig-bridge": "2.1.*"
    },
    "minimum-stability": "dev"    
}

index.php

<?php
require_once __DIR__.'/vendor/autoload.php';

$app = new Silex\Application();

$app['debug'] = true;

$app->register(new Silex\Provider\TwigServiceProvider(), array(
    'twig.path' => __DIR__.'/views' 
));

$app->match('/', function () use ($app) {
    return $app['twig']->render('index.twig');
});

$app->match('/sub', function () use ($app) {
    // This next line is the culprit..
    $app['twig']->addGlobal('foo', 'bar');
    return "This is the sub";
});
 
$app->run();

index.twig

<h1>Silex / Twig render demo</h1>

{{ render('/sub') }}