In my subreport record selection expert I am asking to only include null values for a certain field. I cannot say "" (blank) or trim(thefield)="" because it does not work.

If I only leave the link in the subreport without this extra logic, I get all the dates that are in the subreport. "thefield" is a string version of a date.
There are two fields with dates, If the second field is null, then I want that record.
example
ID#12345 in main report links to id#12345 in subreport,
2 date fiels
firstdate                         seconddatefield
2013-07-11                      2013-07-11 (dont pull this)
2013-08-31                       NULL          (Do select this)

I have tried turning on and off the default checkbox inside the formula editor.

here is what I have.
isnull({vwDrugMissedDrop.StringDropDate}) and
{vwDrugMissedDrop.j001_ID_Number} = {?Pm-vw_tasc2_refer.j001_ID_Number}
 the isnull part not working
if i take that part out I get all the records with dates.