Webservices - filter on empty date

We are using webservices to send data to a website.
One of the webservices is sending item data, with linked Sales Prices.

The sales price table records that must be sent are those with an ending date after today or an empty ending date.

What must I enter on the filter line for exporting the records with an empty ending date?

I tried '0D|21-02-2011..' and all other kind of variations on this statement, but nothing seems to work.

Does anybody know how to filter on an empty date?

Thanks.