There are 2 solutions to this problem (one cause though :) )
1) You probably have defined in the relation that it is allowed to create records over the relation, hence the last row is a row that does not exists yet. You can change the relation not to allow for the creation of records.
2) You can also leave the relation definition as it is and add the script step:
portalrow (select; previous)
If the above is not the case then you may want to enter something in the first portalrow that has not anything entered in a specific field ... then you can do:
Go to Portalrow ( select: first )
If ( isempty (field )
set field (fieldname ; $value)
Go to Portalrow (select: next ; exit after last )
Thanks Menno.. Im going to give it a try...