Calculating A Stored Value

I apologize that I am not a very good VBA programmer yet but I'm really trying....

I am attempting to calculate a value by adding two fields and storing the result into a third field - - My code only works if both of the fields are not null.

Private Sub PartialDepositAmount_AfterUpdate()

Dim PartialDepositAmount As Currency
Dim FinalDepositAmount As Currency
Dim DepositAmount As Currency

Me.DepositAmount = Me.PartialDepositAmount + Me.FinalDepositAmount

End Sub

If either field is null my result is null but I need to be able to add field one (or field two) to a null and store the result in the totals field.

Any suggestions will be greatly appreciated.

JohnJ