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?