[7.x] null signature error during import

I'm just trying to run an import from vbulleting 4.1 to D7:
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Si è verificato un errore HTTP in AJAX. Codice HTTP di risposta: 500 Di seguito le informazioni di debug. Percorso: /batch.htm?id=75&op=do StatusText: Service unavailable (with message) ResponseText: PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'signature' cannot be null: INSERT INTO {users} (pass, mail, created, access, login, signature, name, status, timezone, uid, picture) SELECT u.password AS password, u.email AS email, u.joindate AS joindate, u.lastactivity AS lastactivity, u.lastvisit AS lastvisit, utf.signature AS signature, SUBSTRING(username FROM 1 FOR 60) AS name, 1 AS status, timezoneoffset*60*60 AS timezone, u.userid+:add_to_ids AS uid, IF(ca.userid IS NULL, 0, ca.userid+:pictures_add_to_ids) AS picture FROM {concorsi_vbulletin.user} u LEFT OUTER JOIN {users} du ON u.username = du.name LEFT OUTER JOIN {concorsi_vbulletin.customavatar} ca ON u.userid = ca.userid LEFT OUTER JOIN {concorsi_vbulletin.usertextfield} utf ON u.userid = utf.userid WHERE (du.name IS NULL ) LIMIT 100000 OFFSET 0; Array ( [:add_to_ids] => 1000000 [:pictures_add_to_ids] => 60000000 ) in vbimportapi_import() (linea 1018 di /var/www/sites/concorsipubblici.com/website/sites/all/modules/vbtodrupal/vbimportapi.module).
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PS: please add a 7.x release so we can also open issues against a 7.x release instead of abusing 6.x release.