When a record has a field of type 'object', you can store NULL in it, but once the record is fetched from the database, unserialize will fail (because operating on an empty string) and cause an exception.

The patch for this is quite simple: Change line 1499 in Doctrine_Table from

                        $value = unserialize($value);

to

                        $value = empty($value) ? null : unserialize($value);