Value Summation Problems with C# and Mysql

Question

I’m developing a software that catalogs manga, one of the problems I’m having is with input volumes. What should happen: the user set the numVolumes to 2, that is, he will add 2 more volumes of manga to manga table. But that code isn’t working. numVolues é NumericUpDown e o txtIdtambém.

private void btndicionar_Click(object sender, EventArgs e)
        {
            strMysql = "update manga set volumes = volumes + " + Convert.ToInt(numVolumes.Value) + " where idManga " + txtId.Value + ";";

            MySqlCommand comando = new MySqlCommand(strMysql, con);

            try
            {
                con.Open();
                //eu comentei esses códigos por não achar necessário
                //comando.Parameters.Clear();
                //comando.Parameters.AddWithValue("@soma", numVolumes.Value);


                comando.ExecuteNonQuery();

            }
            catch (Exception erro)
            {
                MessageBox.Show("Ocorreu um erro:\n" + erro);


            }
            finally
            {
                con.Close();

            }