Exception when using Sqlite with Nhibernate for integration tests

Question

I get Exception when using Sqlite with Nhibernate for integration tests: TypeLoadException":generic arguments[0], "system.nullable {system.datetimeOffset} on System.Nullable[T] violates the constraint of type parameter T

I am using below to configure sqllite:

public InMemoryDatabaseTest()
    {
        SessionFactory = Fluently.Configure()
            .Database(SQLiteConfiguration.Standard.InMemory().ShowSql())
            .ProxyFactoryFactory(typeof(ProxyFactoryFactory))
            .Mappings(m => m.FluentMappings
                            .Add(typeof(UserMapping)
            .Conventions.Add(new NormalizedDateTimeUserTypeConversion(),new                     NormalizedDateTimeUserTypeConversion1()))
            .ExposeConfiguration(x => configuration = x)
            .BuildSessionFactory();

        session = SessionFactory.OpenSession();

        SchemaExport export = new SchemaExport(configuration);
        export.Execute(true, true, false, session.Connection, null);
    }

    public void Dispose()
    {
        session.Dispose();
    }

Here is the implementation of NormalizedDateTimeUserType:

public class NormalizedDateTimeUserType : IUserType
{
    private readonly TimeZoneInfo databaseTimeZone = TimeZoneInfo.Local;

    // Other standard interface  implementations omitted ...

    public Type ReturnedType
    {
        get { return typeof(DateTimeOffset); }
    }

    public SqlType[] SqlTypes
    {
        get { return new[] { new SqlType(DbType.DateTime) }; }
    }

    public object NullSafeGet(IDataReader dr, string[] names, object owner)
    {
        object r = dr[names[0]];
        if (r == DBNull.Value)
        {
            return null;
        }

        DateTime storedTime = (DateTime)r;
        return new DateTimeOffset(storedTime, this.databaseTimeZone.BaseUtcOffset);
    }

    public void NullSafeSet(IDbCommand cmd, object value, int index)
    {
        if (value == null)
        {
            NHibernateUtil.DateTime.NullSafeSet(cmd, null, index);
        }
        else
        {
            DateTimeOffset dateTimeOffset = (DateTimeOffset)value;
            DateTime paramVal = dateTimeOffset.ToOffset(this.databaseTimeZone.BaseUtcOffset).DateTime;

            IDataParameter parameter = (IDataParameter)cmd.Parameters[index];
            parameter.Value = paramVal;
        }
    }
}

Implementation of NormalizedDateTimeUserType1:

public class NormalizedDateTimeUserType1 :
NormalizedDateTimeUserType
{
public override Type ReturnedType
{
get { return typeof(DateTimeOffset?); }
}
}