adding allow null to db values posted

Where and how do I add an allow null function to permit users to leave a blank field without getting this error:
[Microsoft][ODBC Microsoft Access Driver] Field 'data.pre_co' cannot be a zero-length string.
I am using a map path method to an access database.
data_add.asp? and I'm unfamiliar with wrappers, etc...so just not sure where to add and what the syntax is for adding. Thank you.