SQLite AutoIncrement always returns 0

I have a android app who use SQLite and sqlite.net pcl library (with xamarin and c#)

This is my table definition (just the id part)

[Table("ConsumoMensual")]
public class ConsumoMensual
{
    [PrimaryKey, AutoIncrement]
    public int? id {get;set;} //also try with int and int?
    ...

I insert data with this code, and returns 1

public void RegistroLlamada(MOD.phoneScrapView v){
        ConsumoMensual c = findRegistroMes (v.FechaLlamada);
        bool update = true;
        if (c == null) {
            update = false;//this is because always get 0
            c = new ConsumoMensual ();
        }

        c.annio = v.FechaLlamada.Year;
        c.mes = v.FechaLlamada.Month;
        c.minutosMismaCompania += minutosMismaCompania (v);
        c.minutosNumerosGratuitos+= ValidoGratuitos (v);
        c.minutosOtraCompania+= minutosOtraCompania (v);
        lock (locker) {
            if (update)
                database.Update (c);//crash here (ID is null)
            else
                database.Insert (c);//returns 1 here
        }
    }

but when i read it, i got zero in the id

    public ConsumoMensual findRegistroMes(DateTime fecha)
    {
        lock (locker) {
            return (from i in database.Table<ConsumoMensual> ()
                    where i.mes == fecha.Month && i.annio == fecha.Year
                    select i).FirstOrDefault ();
        }
    }

this is the interface code

namespace PlanControlDAL
{
    public interface ISQLite
    {
        SQLiteConnection GetConnection();
    }
}

and this is the class who implements the interface public class SQLite_Android : ISQLite { public SQLite_Android () { }

    #region ISQLite implementation
    public SQLite.Net.SQLiteConnection GetConnection ()
    {
        var sqliteFilename = "planControl.db3";
        string documentsPath = System.Environment.GetFolderPath (System.Environment.SpecialFolder.Personal); // Documents folder
        var path = Path.Combine(documentsPath, sqliteFilename);

        // This is where we copy in the prepopulated database
        Console.WriteLine (path);
        if (!File.Exists(path))
        {
            /*var s = Forms.Context.Resources.OpenRawResource(Resource.Raw.totalSQLite);  // RESOURCE NAME ###*/

            // create a write stream

            /*using (StreamReader sr = new StreamReader (Resource.Raw.totalSQLite)) {

                FileStream writeStream = new FileStream (path, FileMode.OpenOrCreate, FileAccess.Write);
                // write to the stream
                ReadWriteStream (sr, writeStream);
            }*/
        }

        var plat = new SQLite.Net.Platform.XamarinAndroid.SQLitePlatformAndroid();
        var conn = new SQLite.Net.SQLiteConnection(plat, path);

        // Return the database connection 
        return conn;
    }
    #endregion

    /// <summary>
    /// helper method to get the database out of /raw/ and into the user filesystem
    /// </summary>
    void ReadWriteStream(Stream readStream, Stream writeStream)
    {
        int Length = 256;
        Byte[] buffer = new Byte[Length];
        int bytesRead = readStream.Read(buffer, 0, Length);
        // write the required bytes
        while (bytesRead > 0)
        {
            writeStream.Write(buffer, 0, bytesRead);
            bytesRead = readStream.Read(buffer, 0, Length);
        }
        readStream.Close();
        writeStream.Close();
    }
}

I'm follow the demos to the letter and i keep getting zero (or null depends if int or int?) what's doing wrong?

Ps. I use the 2.5.1 nuget version of sqlite.net pcl & 2.5.1 sqlite.net.plataform.xamarin.android