If no JOIN is involved in the formation of that RecordSource, numeric fields can't be null. They can be zero. If the control in question is bound to a non-JOIN recordset, it can't be null either, unless somehow NO record is selected - in which case EVERY bound field would be null.
Null occurs in JOINs when a matching record is not found in the JOIN query, and for unbound controls, you can get a null if something doesn't load the control (and it doesn't have an explicit default value). But the important part is that nobody has asked the question yet, so I guess I will.
You say that you get an error when the amount is empty. What, specifically, is the error message?