Problem filling a combo box in java

It turns out that in a mysql database I have some registry and I need to be reflected in a java JComboBox but I can not find the error

I have a class that takes care of the connection to the database and inside it I do that method

106   public void llenarCmbCurso(JComboBox cmbCurso){
 107               cmbCurso.removeAllItems();
 108           try{
 109               String query = "SELECT C_codCurso  FROM t_curso ";
 110               PreparedStatement ps =conect.prepareStatement(query);
 111               ResultSet rest = ps.executeQuery();
 112               
 113           while(rest.next()){
 114                   cmbCurso.addItem(rest.getString("C_codCurso")); 
 115              }
 116           
 117          }catch(SQLException  e ){
 118               System.out.println(e.getMessage());
 119               
 120           }  
 121       }

In JFrame where I want to show such a thing I do the following

 18   Conexion con = new Conexion();
 19    public FMateria() {
 20       initComponents();
 21       this.setLocationRelativeTo(null);
 22       deshabilitar();
 23       con.llenarCmbCurso(cmbCodCurso);
 24    }