Illegal string offset warning

Hi! On creating a new product I see the following error:

Warning: Illegal string offset ‘expires’ in wp-content/plugins/ithemes-exchange/lib/product-features/class.downloads.php on line 428

This is the relevant code:

$meta = get_post_meta( $product_id, '_it-exchange-download-meta', true );
if ( 'limit' == $options['setting'] ) {
    $meta['download-limit'] = $new_value;
} else if ( 'expires' == $options['setting'] ) {
    $meta['expires'] = (boolean) $new_value;
} else if ( 'expire-int' == $options['setting'] ) {
    $meta['expire-int'] = $new_value;
} else if ( 'expire-units' == $options['setting'] ) {
    $meta['expire-units'] = $new_value;
}

However, at least for new products $meta may actually be a string, rather than an array, triggering this error/warning.

Be great to clean it up if possible!

(WP 4.8.1/PHP 7.1)