Bukkit

I user Craftbukkit 1.6.2-R0.1-SNAPSHOT

I have made an BlockPlaceEvent, and i tried to change the block when a player placed it.
I used e.getBlock().setType()
When i debug the plugin, and place the block i wanted to be changed. I was kicked from server for Internal Server Error

      @EventHandler
      public void PlaceBlockEvent(final BlockPlaceEvent e)
      {


      Block g = e.getBlock();

            if (g.getType() == Material.TNT) {

                e.setCancelled(true);
                g.getLocation().getBlock().setType(Material.CAKE);
            }
      }

I have tried all different ways. Either i get kicked, or it will not work