Create XML file

I'm trying to create an XML but it's causing the following error:

  

An exception of type 'System.InvalidOperationException' occurred in System.Xml.dll but was not handled in user code

     

Additional information: There was an error generating the XML document.

Code:

 public Boolean GerarArquivoExportacaoOff(String caminhoArquivo)
        {
            Encoding iso_8859_1 = Encoding.GetEncoding("iso-8859-1");
            StreamWriter streamWriter = new StreamWriter(caminhoArquivo, true, iso_8859_1, 100000);

            List<OFFLINE_TABELAS> offline_TabelasList = getListaTabelasAlteradas(); // Seleciona tabelas alteradas

            foreach (OFFLINE_TABELAS Entity_tabelas in offline_TabelasList)
            {
                var lista = getDados(Entity_tabelas.TABELA); // Obtem entidade a partir do nome da tabela


                System.Xml.Serialization.XmlSerializer serializer = new System.Xml.Serialization.XmlSerializer(lista.GetType());

                using (TextWriter textWriter = new StreamWriter("D:/usuarios.xml"))
                {
                    //Definir o Type e o elemento raiz do XML (usuarios)
                    XmlSerializer ser = new XmlSerializer(lista.GetType(), new XmlRootAttribute("usuarios"));

                    //Serializar o list para o TextWriter e salvar os dados no XML
                    ser.Serialize(textWriter, lista);
                }
}
return true
} 

What's wrong with this?