serial values and odbc

I am working with delphi through odbc to get to Postgres, but I have a
problem:

Everytime I try to make an insertion, I get a message from the odbc driver
saying a Primary key value cannot be null (which is on purpose since I want
postgres to use it serial value properties). Can anyone tell me if there's
something special I have to do on the ODBC configuration, or how do I make an
insertion through odbc?

Thanx.