Problem with ID generation for more than 50 objects created in one transaction

[szpak]

@Entity
public class Image {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO, generator = "image_sequence")
    @SequenceGenerator(name = "image_sequence", sequenceName = "image_sequence")
    private Long id;

    @Column
    String name;

   (...)
}

Creating more than 50 images in one transaction (on freshly initialized database) results in:

java.sql.SQLException: ERROR: duplicate key value violates unique constraint "image_pkey"
  Detail: Key (id)=(2) already exists. Query: INSERT INTO Image
(id, name)
VALUES
    (?, ?) Parameters: [2, foo]

Batoo took 50 numbers from sequence and had problem when used all of them? It works fine with allocationSize set to 1. BatooJPA 2.0.1.0-RTM, PopstgreSQL 9.2.

Sample test code snippet (in Spock with SpringData):

(...)
    @Transactional
    def "should allow to insert over 50 images in one transaction"() {
        setup:
            for (i in 0..50) {
                imageRepository.save(new Image("foo"));
            }
        expect:
            imageRepository.count() == 51
    }