Problem writing ComboBox data in Postgresql

First is the code of the bank I'm using:

CREATE TABLE perguntas (
    cod_pergunta SERIAL PRIMARY KEY NOT NULL,
    pergunta VARCHAR(500),
    opcao_um  VARCHAR(500),
    opcao_dois VARCHAR(500),
    opcao_tres VARCHAR(500),
    opcao_quatro VARCHAR(500),
    opcao_correta INTEGER,
    IDcategoria INTEGER,
    CONSTRAINT fk_categoria FOREIGN KEY (IDcategoria) REFERENCES categoria(cod_categoria)
);

CREATE TABLE categoria (
    cod_categoria SERIAL PRIMARY KEY NOT NULL,
    categoria VARCHAR(15),
    descricao VARCHAR(140)
);

I can record the values in the questions table but the value of the 'category' combobox is writing wrong when I try to write the first value.

Image of the form:

Savebuttoncode:

privatevoidbtnGravar_Click(objectsender,EventArgse){//Verificaqualradiobuttonestáselecionadointvalor;valor=0;if(rbCorreta1.Checked==true)valor=1;elseif(rbCorreta2.Checked==true)valor=2;elseif(rbCorreta3.Checked==true)valor=3;elseif(rbCorreta4.Checked==true)valor=4;elseMessageBox.Show("Selecione a resposta correta!");


//Verifica qual o valor do combobox está selecionado e guarda o ID para gravar
            string IndexSelecionado = cbCategoria.SelectedIndex.ToString();



            string str = "Host=127.0.0.1;Username=postgres;Password=adm;Database=dbquiz";
            string gravarsql = "INSERT INTO perguntas (pergunta, opcao_um, opcao_dois, opcao_tres, opcao_quatro, opcao_correta, idcategoria) " + " VALUES ('" + txtPergunta.Text + "', '" + txtResposta1.Text + "', '" + txtResposta2.Text + "', '" + txtResposta3.Text + "', '" + txtResposta4.Text + "', '" + valor + "', '"+ IndexSelecionado + "');";
            Npgsql.NpgsqlConnection con = new Npgsql.NpgsqlConnection(str);
            Npgsql.NpgsqlCommand cmd = new Npgsql.NpgsqlCommand(gravarsql, con);
            cmd.CommandType = CommandType.Text;
            con.Open();

            try
            {
                int n = cmd.ExecuteNonQuery();
                if (n > 0)
                {
                    MessageBox.Show("Efetuado!");
                }

            }
            catch (Exception ex)
            {
                MessageBox.Show("Error: " + ex.ToString());
            }
            finally
            {
                con.Close();
            }
        }

The error that appears is this below, but it happens only when I want to record the 1st value of the combobox because the rest of it is to write only that it writes the wrong index:

Ex.

Categories: 1-Test, 2-Foo, 3-Stack; When I select 2, it saves 3.

CodetopopulatetheCombobox:

stringstr="Host=127.0.0.1;Username=postgres;Password=adm;Database=dbquiz";
            Npgsql.NpgsqlConnection con = new Npgsql.NpgsqlConnection(str);
            con.Open();

            try
            {
                string sql = "SELECT categoria.cod_categoria, categoria.categoria FROM categoria;";
                Npgsql.NpgsqlCommand cmd = new Npgsql.NpgsqlCommand(sql, con);
                Npgsql.NpgsqlDataReader reader = cmd.ExecuteReader();
                DataTable dt = new DataTable();
                dt.Load(reader);

                this.cbCategoria.DataSource = dt;

                this.cbCategoria.DisplayMember = "categoria";
                this.cbCategoria.ValueMember = "cod_categoria";

                reader.Close();
                reader.Dispose();

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            finally
            {
                con.Close();
                con.Dispose();
            }