Condition ChooseFromList doesn't work

Hi

On the AP Invoice Form i added to the ChooseFromList a new condition. This works with the following code fine.

SAPbouiCOM.ChooseFromListCollection cflc = parentForm.ChooseFromLists;
SAPbouiCOM.ChooseFromList cfl = cflc.Item("2");
SAPbouiCOM.Conditions conds = cfl.GetConditions();
SAPbouiCOM.Condition cond = conds.Add();
cond.Alias = "ISRBillID";
cond.Operation = SAPbouiCOM.BoConditionOperation.co_EQUAL;
cond.CondVal = "1234";
cfl.SetConditions(conds);

This ChoosFromList give me all BusinessPartners with ISRBillID back.

When i want all BusinessPartners with no ISRBillID, it doesn't work.

SAPbouiCOM.ChooseFromListCollection cflc = parentForm.ChooseFromLists;
SAPbouiCOM.ChooseFromList cfl = cflc.Item("2");
SAPbouiCOM.Conditions conds = cfl.GetConditions();
SAPbouiCOM.Condition cond = conds.Add();
cond.Alias = "ISRBillID";
cond.Operation = SAPbouiCOM.BoConditionOperation.co_EQUAL;
cond.CondVal = "";
cfl.SetConditions(conds);

Does anybody have an idea?