Paypal address invalid without postcode

I'm building a website using Hikashop(great extension) and am using Paypal standard as my payment method. If the customer doesn't enter a postcode into the address field, I get the following message on the paypal site:

"Unable to process payment. Please contact the merchant as the postal address provided by the merchant is invalid, and the merchant has requested that your order must be delivered to that address."

Entering a postal code fixes this.

Is it possible to make Paypal more relaxed about the postcode? Or if not, is there a way to make the postcode a required field in Hikashop?

Here are my versions:

Joomla 1.7.1
Hikashop starter 1.5.3

Here are my paypal settings:

Thanks very much for your speedy response! The downside of not using 'address override' is that the customer will have to enter their address twice - is this right?

I've been doing some testing and it seems if you don't use the address override then Paypal does ask the user to enter their address again.

I think I'll make the postcode a mandatory field (I checked a Virtuemart store and this is what they do).

Thanks very much for your help.