Fatal error: Uncaught PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'isbn' cannot be null in /var/www/html/shelf_verification.php:219 Stack trace: #0 /var/www/html/shelf_verification.php(219): PDOStatement->execute() #1 {main} thrown in /var/www/html/shelf_verification.php on line 219