Hibernate and ORA-01400: cannot insert NULL

I have a pretty simple script, one table, one sequence for automatically generating primary keys:

CREATE TABLE foo (event_id NUMBER(19,0)....
CREATE SEQUENCE event_seq

Part of the hibernation display is as follows:

...
<id name="id" column="EVENT_ID" type="long">
   <generator class="native">
      <param name="sequence">event_seq</param>
   </generator>
</id>
...

It works well without problems for a long time. But recently I got this:

ORA-01400: cannot insert NULL into ("TESTING"."FOO"."EVENT_ID")

, Oracle EVENT_ID? ? -, 64- Oracle, ? , . , , , , .

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Oracle 10.2.0.1.0 64bit 10.2.0.1.0 32- . 11.2.0.2.0 . VM, 32- 64- .