Cannot find an appropriate conversion for valueMember which im trying to insert based on the picked display member

Question

//This is the insertion code

    private void button4_Click(object sender, EventArgs e)
    {

        SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Andrej\Desktop\B9PROBA\B9\B9\Database5.mdf;Integrated Security=True;Connect Timeout=30");
        SqlCommand com = new SqlCommand("INSERT INTO ARB5Stadion(StadionID,Naziv,Adresa,Kapacitet,BrojUlaza,GradID ) VALUES ('" + textBox1.Text + "','" + textBox2.Text + "','" + textBox3.Text + "','" + textBox4.Text + "','" + textBox5.Text + "','" + comboBox1.ValueMember + "')", conn);
        
        try
        {
            conn.Open();
            SqlDataAdapter da = new SqlDataAdapter(com);
            DataSet ds = new DataSet();
            da.Fill(ds);
            MessageBox.Show("Uspesno Uneti Podaci");


        }
        catch (Exception e1) { MessageBox.Show(e1.Message); }
        finally { conn.Close(); }

//And this is the code for the combobox, which reads from one datatable ,using the the id value as an value member and the name as a display memeber.Im essentialy trying to insert the GradID primary key of the ARB5Grad Table into StadionID where GradID is a foreign key

    private void Stadion_Load(object sender, EventArgs e)
    {
        SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Andrej\Desktop\B9PROBA\B9\B9\Database5.mdf;Integrated Security=True;Connect Timeout=30");
        conn.Open();
        SqlCommand sc = new SqlCommand("SELECT GradID,Grad FROM ARB5Grad ", conn);
        SqlDataReader reader = sc.ExecuteReader();
        DataTable dt = new DataTable();
        dt.Columns.Add("GradID", typeof(int));
        dt.Columns.Add("Grad", typeof(string));
        dt.Load(reader);

        comboBox1.ValueMember = "GradID";
        comboBox1.DisplayMember = "Grad";
        comboBox1.DataSource = dt;