Blank field in column

From: Bill (anonymous_at_discussions.microsoft.com)
Date: 06/30/04 

Date: Wed, 30 Jun 2004 16:05:36 -0700

I am using the following query:

DELETE FROM ORDER_HEADER
WHERE (INVOICE_NUMBER IS NULL) OR (INVOICE_NUMBER = '0')

This works fine but doesn't remove the records where the
INVOICE_NUMBER field is blank.

Is there a way to identify a blank or empty field?