diff --git a/advtrains/poconvert.lua b/advtrains/poconvert.lua
index 3075282..74f962e 100644
--- a/advtrains/poconvert.lua
+++ b/advtrains/poconvert.lua
@@ -13,7 +13,7 @@ do
 	local function replace_single(pfx, c)
 		local pl = #pfx
 		if pl % 2 == 0 then
-			return pfx:sub(1, pl/2) .. c
+			return string.sub(pfx, 1, pl/2) .. c
 		end
 		return string.sub(pfx, 1, math.floor(pl/2)) .. (schartbl[c] or c)
 	end