Execute scalar c#

Question

If put in text box username (a) and password (a) app say invalid column name "a"

 

Line 10 problem....some help?

private void prijava()
        {


            SqlConnection myConnection = new SqlConnection(cs);
            String command = "SELECT id FROM osobe_korisnici where korisnicko_ime = " + korisnikTextBox.Text + "";
            myConnection.Open();
            SqlCommand myCommand = new SqlCommand(command, myConnection);

            korisnikId = Int32.Parse(myCommand.ExecuteScalar().ToString());