PHP notice on custom field

In log, we get this PHP message: is_internal_meta_key was called incorrectly. t refers to the added custom field in the custom invoice.

The rest of the PHP message:

De generieke meta-methoden add/update/get mogen niet
worden gebruikt voor interne meta-data, waaronder “_billing_company”. Gebruik setters en getters.

<div class="billing-vat"><?php $this->custom_field('billing_company', 'Bedrijf:'); ?></div><div class="billing-vat"><?php $this->custom_field('billing_eu_vat_number', 'BTW:'); ?></div>

The way of adding the $this->custom_field is described in your documentarion, but is this still valid?

Thanks!