How to avoid DROP DEFAULT statements with Doctrine 2 Migrations diff on first run?

I had an existing PostgreSQL database with a table created like this:

CREATE TABLE product (id SERIAL PRIMARY KEY, name VARCHAR(100) DEFAULT NULL)

This table is described in a YML Doctrine2 file within a Symfony2 project:

Acme\DemoBundle\Entity\Product:
    type: entity
    table: product
    fields:
        id:
            id: true
            type: integer
            nullable: false
            generator:
                strategy: SEQUENCE
        name:
            type: string
            length: 100
            nullable: true

When I run for the first time the Doctrine Migrations diff task, I should get a versioning file with no data in the up and down methods. But what I get instead is this :

// ...

class Version20120807125808 extends AbstractMigration
{
    public function up(Schema $schema)
    {
        // this up() migration is autogenerated, please modify it to your needs
        $this->abortIf($this->connection->getDatabasePlatform()->getName() != "postgresql");

        $this->addSql("ALTER TABLE product ALTER id DROP DEFAULT");
    }

    public function down(Schema $schema)
    {
        // this down() migration is autogenerated, please modify it to your needs
        $this->abortIf($this->connection->getDatabasePlatform()->getName() != "postgresql");

        $this->addSql("CREATE SEQUENCE product_id_seq");
        $this->addSql("SELECT setval('product_id_seq', (SELECT MAX(id) FROM product))");
        $this->addSql("ALTER TABLE product ALTER id SET DEFAULT nextval('product_id_seq')");
    }
}

Why are differences detected? How can I avoid this? I tried several sequence strategies with no success.