Set Edit box value with null value

Using SUP 2.2.

I have an MBO with a parameter defaulted value is <NULL>.  So if this parameter is left alone, it would bring all the data back without any specific filter.

However, the parameter, as you might guess is coming from a drop down list which has static values.  The MBO works well when the selected value from the list has a specific value.  But I do have an option in the drop down with "All", which I need to some how set the drop down list with <NULL>.  I've tried setting the field with empty quotation marks, with null and even <NULL>.  But no data is returned.  I am positive that the query does work and should have returned data, that I am very sure off.

So my question is, how can I set this drop down box with null value so that the MBO will work?  Thanks.