Google app scripts. system groups names issue

Why this do not work? i need to move "other contacts" to "my contacts"

  var activeSheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();

  var group  = ContactsApp.getContactGroup('Other Contacts');
  var contacts = ContactsApp.getContactsByGroup(group);
  var mainGroup = ContactsApp.getContactGroup("System Group: My Contacts");

  for (var i in contacts) {
    mainGroup.addContact(contacts[i]);
  }