Rails - Mar 2006 - undefined local variable or method `errors'' for Customer:Class

If I add the following line to my customer model:
errors.add(:shipping_address_postcode, "has invalid format") unless 
:shipping_address_postcode =~ /[0-9]*/

I get an error when trying to save a record:

undefined local variable or method `errors'' for Customer:Class

I got this example from the Agile book.  Anyone know what I''ve missed?

Thanks,
Dan

Dan Harper wrote:
> If I add the following line to my customer model:
> errors.add(:shipping_address_postcode, "has invalid format")
unless
> :shipping_address_postcode =~ /[0-9]*/
>
> I get an error when trying to save a record:
>
> undefined local variable or method `errors'' for Customer:Class
It''s OK, I figured it out.  I needed to put this line in a "def 
validate" section.

Dan