Update a field to null

Is it possible using Expression Builder formulas to update an integer field to a Null value?
I tried with <No Values>, <Null>, Null .... but it is not working.
Example:
IIf([<plDD_Payment_Frequency_ITAG>] = 'Annual', 1, IIf([<plDD_Payment_Frequency_ITAG>] = 'Quarterly', 4, IIf([<plDD_Payment_Frequency_ITAG>] = 'Monthly', 12, NULL)))

Thanks