Query Criteria to Show Null Records if it meet the IIF Condition

[Mohsin Malik]

Hi there, I am setting up the criteria for my Query tfrom form to show the result if it meets the criteria of showing payment but not yet paid to show me all the records having [Total Net Paid] is null, "Required but Not Paid" is the requirement for which whose result would be showing all record whose Total Net paid Result is null, i dont know how to do this, please help me in this matter.

IIf([Forms]![FY]![st]="All",[Total Net Paid],IIf([Forms]![FY]![st]="Required",[Total Net Paid],IIf([Forms]![FY]![st]="Required but Not Paid",".................")))

".................", Please help me what can i put here to show the Null records if the Form St field is set to "Required but Not Paid".

Thank you