When moving data from Paradox tables to firebird. I always get an INTEG_23
constrating error on the following table:

CREATE TABLE Owner /* Wp */
(
OwIDSQ INTEGER NOT NULL ,
CityVBSQ INTEGER NOT NULL ,
OwVBSQ INTEGER ,
OwName VARCHAR(120) NOT NULL ,

PRIMARY KEY (OwIDSQ) ,
FOREIGN KEY (CityVBSQ) REFERENCES City (CityIDSQ) ON DELETE CASCADE ,
FOREIGN KEY (OwVBSQ) REFERENCES Whitepage (OwIDSQ) ON DELETE CASCADE
) ;

OwVBSQ may reference another record in the same table but must not. I gave
OwVBSQ a NOT NULL constraint so it could take NULL values for those records
where this self referencing does not apply. But as soon as I enter a record
where OwVBSQ is NULL I get an error message. What is wrong ?