Getting error: "Invalid use of Null"

Hi everybody!

When I try to convert an access database with the database migration 
wizard, I receive the message "94: Invalid use of Null". I sure that I 
do all the steps to database conversion right, what can be wrong? (NOTE: 
the odbc converison method is ok, but - as expected - it don't create 
the foreign keys of the database). I'm using the MS-Access 97 and 
PgAdmin v.1.2.0 with Postgresql v 7.1.3 for MS-Windows.

Any comments is going to be a big help!

Thanks.

MARCELO ANDRADE
Belem-Para-Amazonia-Brasil