Can't make VAT number field optional in 1.6.0.6

[ivan.ps]

When a user has to fill up an address (Add my first address or during the checkout process or even when adding additional addresses) no matter if I fill the "Company" field I always get the error:

The field vat_number is required.

The vat number field appear just if I fill the Company field with something. A user can hardly even find a workaround since it won't be able to see the VAT field unless he fills the Company field.

This happens even if:
- "Do you need a tax identification number?" is set to no
- and even if I delete vat_number from the list in Localization -> Countries -> Address format.

So no matter if there is an accessible vat_number field in the address form I always get the error.

The only way to get rid of the error is to fill the Company and VAT number.