if your list of values has a PK (primary key) and a description i.e.:
Code:
Opt_ID Opt_Desc
1 N/A
2 Description 1
3 Description 2
If you are using a BOUND form you must populate the option field with the Opt_ID field, not the description.
Similarly, if you are using an UNBOUND form you must look up the value for N/A and populate it before appending the record.
Just be aware, if you are using a BOUND form (a form tied directly to a table or query) if you populate the value as soon as you get to a new record a new record is created that will essentially be blank with the exception of the fields you've forced to have a default value. In other words, I would not populate the 'blank' fields with N/A until you navigate away from the record rather than as soon as you open the record.