使用SESSION变量为foreach（）提供的参数无效

I'm doing a cart for a web configurator and I've a problem. I got this:

if (!empty($_SESSION["pedido"]) && is_array($_SESSION["pedido"])) {
        echo "Ha seleccionado:";
        foreach($_SESSION['pedido'] as $prod => $unidades) {
            echo "$unidades unidades de $prod";

When I execute it on MAMP, it works well, but on my server, it shows the error: Warning: Invalid argument supplied for foreach() in [lineOfCode]. I've tried to search a solution, for example, casting the $_SESSION['pedido'] to array, and it works, until I got the $prod key, that is not the correct (its returns a 0 instead of the product key), and I need that key for a switch operation. Thanks.