Outlook-Items Find method

someone uses the outlook-method Find to retrive single items (calendar,
task, ..)? My problem is that it doesn't work on empty properties. Something
like " [BillingInformation] = '' "  does not return any item also if there
are a lot of items without BillingInformation. Is there any keyword (like
Empty, Nothing, Null) or any other work around (I tried just with
"[BillingInformation] < '!' ", but always 0 items returned).
Could be the right place for this message would be the MS-Newsgroup for
Office or Outlook but first I tried here having more confidence in delphi