Navision Request Form - Blank Field

I created a dataport. When the dataport runs, the request form pops up, letting me enter fields to filter the dataport. I wish to choose the "Currency Code" field and only show records which have a blank currency code. That's because the blank currency code signifies the local currency. But when I leave the "Currency Code" field blank, it just ignores the "Currency Code" field altogether and shows me all the records. On the other hand, if I choose the "EUR" currency code, it shows me only records with the EUR currency code.

Is there any way for me to filter a dataport via the request form so that it only shows records with a blank "currency code"? Thanks.