PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value

step to reproduce:
- create a relation "R" between node of type "A" and node of type "B".
- add to node of type "A" a field with "relation select" widget that target node of type "B", illimitate value allowed
- add to the relation itself a field of type entityreference, widget autocomplete, 2 value allowed

NOW
if you i add a content "A", the form is:
autocomplete B - relation endopoint
---select 1, field of the relation
---autocomplete
---autocomplete (second value)
(+ add another endpoint)

BUT
if you left blank one of the autocomplete field, you get this error:

PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: '' for column 'field_avvnodo_target_id' at row 2: INSERT INTO {field_data_field_avvnodo} (entity_type, entity_id, revision_id, bundle, delta, language, field_avvnodo_target_id) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6), (:db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13), (:db_insert_placeholder_14, :db_insert_placeholder_15, :db_insert_placeholder_16, :db_insert_placeholder_17, :db_insert_placeholder_18, :db_insert_placeholder_19, :db_insert_placeholder_20); Array ( [:db_insert_placeholder_0] => relation [:db_insert_placeholder_1] => 60 [:db_insert_placeholder_2] => 85 [:db_insert_placeholder_3] => causa_soggetti [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => 256 [:db_insert_placeholder_7] => relation [:db_insert_placeholder_8] => 60 [:db_insert_placeholder_9] => 85 [:db_insert_placeholder_10] => causa_soggetti [:db_insert_placeholder_11] => 1 [:db_insert_placeholder_12] => und [:db_insert_placeholder_13] => [:db_insert_placeholder_14] => relation [:db_insert_placeholder_15] => 60 [:db_insert_placeholder_16] => 85 [:db_insert_placeholder_17] => causa_soggetti [:db_insert_placeholder_18] => add_more [:db_insert_placeholder_19] => und [:db_insert_placeholder_20] => A ) in field_sql_storage_field_storage_write() (linea 448 di /home/tcbkitrp/public_html/modules/field/modules/field_sql_storage/field_sql_storage.module).