Multiple checkbox in wordpress fires invalid argument for in array when data is still not saved

I'm trying to make multiple checkbox work for custom meta based in language values. I founded this code in another question and works fine except for the first time you run the code when data is not saved yet returns " in_array() expects parameter 2 to be array string given". I suppose it's because add_meta_data() it's not working in the way intended-if postmeta is empty- but I don't see why or how to make it work. Thanks in advance.

function futbol_meta_box( $post ) {
// Get post meta value using the key from our save function in the second paramater.
$futbol_meta = get_post_meta($post->ID, '_futbol-meta-box', true);

?>
    <input type="checkbox" name="futbol-meta-box[]" value="en" <?php echo (in_array('en', $futbol_meta)) ? 'checked="checked"' : ''; ?> />Inglés
    <br>
    <input type="checkbox" name="futbol-meta-box[]" value="fr" <?php echo (in_array('fr', $futbol_meta)) ? 'checked="checked"' : ''; ?> />Francés
    <br>
    <input type="checkbox" name="futbol-meta-box[]" value="es" <?php echo (in_array('es', $futbol_meta)) ? 'checked="checked"' : ''; ?> />Español<br>
<?php}

add_action( 'save_post', 'save_futbol_meta_box' );
function save_futbol_meta_box() {

global $post;
// Get our form field
if(isset( $_POST['futbol-meta-box'] )) {
    $futbol = $_POST['futbol-meta-box'];
    $old_meta = get_post_meta($post->ID, '_futbol-meta-box', true);
    // Update post meta
    if(!empty($old_meta)){
        update_post_meta($post->ID, '_futbol-meta-box', $futbol);
    } else {
        add_post_meta($post->ID, '_futbol-meta-box', $futbol, true);
    } 
} }