Need Date Field to be Blank if Other Field is Blank

My [Next Reminder Date] formula is:
If(not IsNull([Next Reminder # Days before Exp]), [Expires]-Days([Next Reminder # Days before Exp]), null), 

but this field is not appearing blank if there is no value in the [Next Reminder # Days before Exp] field...instead if is display the date in the [Expires] field.

How can I have this date field be blank if [Next Reminder # Days before Exp] is also blank?

Thank you!