diff --git a/src/luarocks/tools/tar.lua b/src/luarocks/tools/tar.lua
index 6db1334a..49139615 100644
--- a/src/luarocks/tools/tar.lua
+++ b/src/luarocks/tools/tar.lua
@@ -51,11 +51,11 @@ local function octal_to_number(octal)
    local number = 0
    octal = octal:gsub("%s", "")
    for i = #octal, 1, -1 do
-      local digit = math.tointeger(octal:sub(i, i))
+      local digit = tonumber(octal:sub(i, i))
       if not digit then
          break
       end
-      number = number + (digit * math.tointeger(8 ^ exp))
+      number = number + (digit * (8 ^ exp))
       exp = exp + 1
    end
    return number