diff --git a/src/main/resources/scripts/world.lua b/src/main/resources/scripts/world.lua
index 0accd486..f3af6284 100644
--- a/src/main/resources/scripts/world.lua
+++ b/src/main/resources/scripts/world.lua
@@ -90,6 +90,8 @@ local entityTypes = {
 
 local function entityTypeNamesToIntegers(input, fullName)
 	if input then
+		local output = {}
+
 		for i, v in ipairs(input) do
 			-- could have used string.lower, but original engine does case-sensitive comparison here
 			-- so we can save quite a lot cpu cycles for ourselves by not doing string.lower
@@ -99,10 +101,10 @@ local function entityTypeNamesToIntegers(input, fullName)
 				error('invalid entity type ' .. tostring(v) .. ' for ' .. fullName .. ' in types table at index ' .. i, 3)
 			end
 
-			input[i] = lookup
+			output[i] = lookup
 		end
 
-		return input
+		return output
 	end
 end