>>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.

Very strange. Do both columns Numero and NoClient in these tables match in types ?

Also do you have NULLs in these fields in any of these tables? Or any abnormal data?