error get names of inbox sub folder in outlook

private void comboBoxInboxFolders_SelectedIndexChanged(object sender, EventArgs e)
     {
         LoadEmailFolderCombo(comboBoxInboxFolders);
     }

     private void LoadEmailFolderCombo(ComboBox nbFolders)
     {

         OutLook._Application outlookObj = new OutLook.Application();
         OutLook.MAPIFolder emailFolder = (OutLook.MAPIFolder)
          outlookObj.Session.GetDefaultFolder(OutLook.OlDefaultFolders.olFolderInbox);

         if (!nbFolders.Items.Contains("Default"))
         {
             nbFolders.Items.Add("Default");
         }
         foreach (OutLook.MAPIFolder subFolder in emailFolder.Folders)
         {
             if (!nbFolders.Items.Contains(subFolder.Name))
             {
                 nbFolders.Items.Add(subFolder.Name);
             }
         }

     }