If I use markDirty() to save inventory changes, it get me fail. What can make problems? May be I must import library, or something else?

Part of code where I use it

if (entity.capabilities.isCreativeMode) {
                         if (entity.inventory.getCurrentItem() != null && entity.inventory.getCurrentItem().getItem() == mcreator_expand1.block) {
                              if (true) {
                                   if (entity instanceof EntityPlayer)
                                        ((EntityPlayer) entity).inventory.setInventorySlotContents(selSlot, new ItemStack(mcreator_expand50.block, 1));
                                        stopExpand = 1;
                                        markDirty();
                              }
                         }
                       }

 

I solve the problem with fail

if (entity.capabilities.isCreativeMode) {
	if (entity.inventory.getCurrentItem() != null && entity.inventory.getCurrentItem().getItem() == mcreator_expand1.block) {
		if (true) {
			if (entity instanceof EntityPlayer)
			((EntityPlayer) entity).inventory.setInventorySlotContents(selSlot, new ItemS(mcreator_expand50.block, 1));
			((EntityPlayer) entity).inventory.markDirty();
		}
	}
}

but it isn`t help with my problem...