diff --git a/storage/itemstack.lua b/storage/itemstack.lua
index fe7c833..111c513 100644
--- a/storage/itemstack.lua
+++ b/storage/itemstack.lua
@@ -122,7 +122,7 @@ function ItemStack:hasChanged(listObj, thorough)
 end
 
 function ItemStack:_modify(countDelta, stack)
-  local newCount = self.count + countDelta
+  local newCount = self:getCount() + countDelta
   if newCount < 0 then
     error("ERROR: New stack count is negative: "..newCount)
   end