Nhibernate ignoring unsaved-value with Guid (SQL Server CE) - nhibernate

Question

This is my mapping:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="DomainModel" namespace="DomainModel">
<class name="Cliente" table="Clientes">
<id name="Id" column="ID" type="guid" unsaved-value="00000000-0000-0000-0000-000000000000">
<generator class="guid.comb" />
</id>
<property name="Nombre" column="Nombre" not-null="true" type="string" length="50" />
<property name="Direccion" column="Direccion" not-null="false" type="string" length="75" />
<property name="Telefono" column="Telefono" not-null="false" type="string" length="15" />
<property name="Email" column="Email" not-null="false" type="string" length="50" />
<property name="FechaAlta" column="FAlta" not-null="true" type="DateTime" />
<list name="Pedidos" cascade="all-delete-orphan">
<key column="Cliente"/>
<index column="FechaEntrada"/>
<one-to-many class="Pedido"/>
</list>
</class>
</hibernate-mapping>
When I try to insert a new "Cliente" (Customer) into the database, NHibernate generates instead an UPDATE command, looking to update the customer with ID 00000000-0000-0000-0000-000000000000. As it doesn't exist, I naturally get the exception:
InnerException: NHibernate.StaleStateException
Message="Unexpected row count: 0; expected: 1"
I'm guessing this has to be a noob NHibernate error, but I've been checking my mapping with other guid example mappings on the web and I don't see my mistake.
Thanks!
PS.- As requested, this is the code I use to do the insertion:
// METHOD 1
//BusinessTransaction bt = new BusinessTransaction(sesion);
//bt.Attach(cliente);
//bt.Commit();
//bt.Close();
// METHOD 2
ITransaction trans = sesion.BeginTransaction();
sesion.SaveOrUpdate(cliente);
trans.Commit();
sesion.Flush();
sesion is an ISession object created with the BuildSessionFactory method of NHibernate Configuration. Method 1 provokes the error described, an UPDATE when it should be doing an INSERT, while Method 2 works, however (I'm an idiot and was looking a the wrong database file).
PPS.- For the sake of testing I've changed the guid fields for auto-incremented int identity fields, but my problems are the same.
PPPS.- Just in case, this are the contents of my hibernate.cfg.xml file:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
<session-factory>
<property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
<property name="dialect">NHibernate.Dialect.MsSqlCeDialect</property>
<property name="connection.driver_class">NHibernate.Driver.SqlServerCeDriver</property>
<property name="connection.connection_string">Data Source=Pedidos.sdf</property>
<property name="show_sql">true</property>
<property name="proxyfactory.factory_class">NHibernate.ByteCode.LinFu.ProxyFactoryFactory, NHibernate.ByteCode.LinFu</property>
<mapping assembly="PedidosDomainModel" />
</session-factory>
</hibernate-configuration>

Answer 1

Here's one solution: Map the Id (that's how I map it in Fluent, you will sort out an equivalent in XML mapping)
Id(x => x.Id).GeneratedBy.Assigned();
And before saving you can do something like this:
if (client.HasEmptyId())
{
client.Id = Guid.NewGuid();
session.Save(client);
}
else
{
session.Update(client);
}