Usa la funzione Nz con la quale puoi assegnare un valore di default (in questo caso ad esempio 0 se il valore è null)

Es:

Maths involving a Null usually results in Null. For example, newbies sometimes enter an expression such as this in the ControlSource property of a text box, to display the amount still payable:

=[AmountDue] - [AmountPaid]

The trouble is that if nothing has been paid, AmountPaid is Null, and so this text box displays nothing at all.

Solution
Use the Nz() function to specify a value for Null:

= Nz([AmountDue], 0) - Nz([AmountPaid], 0)