Changing back button text makes back arrow disappear

When I change the title of NavigationController's back button using the following code, the "back arrow" is missing in iOS 7. Under iOS 6, the problem is almost the same : the back button lose it's "back" appearance

iOS 7 :

iOS 6 :

My code :

public SpotViewControllerController (PageSpots pageSpots) : base (UITableViewStyle.Grouped)
{
   UIBarButtonItem button = new UIBarButtonItem("Zones", UIBarButtonItemStyle.Done, (sender, args) =>
   {
      NavigationController.PopViewControllerAnimated(true);
   });
   NavigationItem.SetLeftBarButtonItem (button, true);
}

Could not find any property to set to fix this. What is wrong with this code?