json_encode不会打印任何内容

I have the follow code which doesn't print anything:

$array = array();
while ($res = $query->fetch_array(MYSQLI_ASSOC)) {
    array_push($array, array(
        'value' => $res['pregunta'],
        'tokens' => explode(' ',$res['pregunta']),
        'url' => $res['id-cadena']
        ));
}

echo json_encode($array);

But when I use print_r it works correctly:

    Array
(
    [0] => Array
        (
            [value] => ¿Cuál es la obra de Pablo Picasso basada en triangulos?

etc...

Have somebody had the same problem? Thanks a lot!