>>Please post the failing command. Perhaps it has nothing to do with the NULL settings.
>
>I succeeded to diminish the command to this simplest format:
>
>

>SELECT Client.Numero FROM Client
> INNER JOIN Invoice ON Client.Numero=Invoice.NoClient
> WHERE Client.Numero=1
>

>
>As long as the INNER JOIN is in there, I will get the error. I checked the joined table. There are no missing primary keys. All records have one and they are all unique.

Hmmm, this is the easiest command. You're saying, that removing the orphans didn't fix the problem?

I would probably understand this error if you selected * instead of just one field.