Error when attempting to pass custom field

I am using this code in functions.php to try and pass a custom checkout field to Stripe.

add_filter( 'wc_stripe_payment_metadata', 'stripe_payment_metadata_filter_callback', 10, 3 );
function stripe_payment_metadata_filter_callback( $metadata, $order, $prepared_source ) {
    // Here below define your custom field meta key (as it's saved in wp_postmeta DB table)
    $metadata = 'custom_meta_key';

    $metadata[ __( 'Custom Label Text (or meta key)', 'woocommerce-gateway-stripe' ) ] = $order->get_meta($meta_key);

    return $metadata;
}

I added my custom meta key ‘billing_child_name’ where it says:
$metadata = ‘custom_meta_key’;

and also in the line below that:
$metadata[ __( ‘Custom Label Text (or meta key)’,

I tried it first with the meta key and then with the custom label text. Each time, I get the following error:

====Stripe Version: 4.5.3====
====Start Log====
Error: stdClass Object
(
    [error] => stdClass Object
        (
            [message] => Invalid object
            [param] => metadata
            [type] => invalid_request_error
        )

)

What am I doing wrong?

Thanks.