diff --git a/compacting-storage.lua b/compacting-storage.lua
index 3c96bfa..b3edf3f 100644
--- a/compacting-storage.lua
+++ b/compacting-storage.lua
@@ -46,12 +46,13 @@ function sum_items (inv)
     local result = {}
     for _, type in ipairs(ITEM_TYPES) do
         result[type] = {}
+        for _, form in ipairs(PRIORITY) do
+            result[type][form] = 0
+        end
     end
     for slot, item in pairs(inv.list()) do
         local type, form = item.name:match".-:(%l-)_(%l*)"
-        if result[type][form] == nil then
-            result[type][form] = item.count
-        else
+        if type ~= nil and form ~= nil then
             result[type][form] = result[type][form] + item.count
         end
     end