diff --git a/lib/microwaveprop/buildings/ms_footprints.ex b/lib/microwaveprop/buildings/ms_footprints.ex
index 1605a902..ac0a5f9d 100644
--- a/lib/microwaveprop/buildings/ms_footprints.ex
+++ b/lib/microwaveprop/buildings/ms_footprints.ex
@@ -58,7 +58,7 @@ defmodule Microwaveprop.Buildings.MsFootprints do
   end
 
   defp encode_quadkey(tx, ty, zoom) do
-    Enum.reduce(zoom..1, "", fn i, acc ->
+    Enum.reduce(zoom..1//-1, "", fn i, acc ->
       mask = Bitwise.bsl(1, i - 1)
       digit = digit_for(tx, ty, mask)
       acc <> Integer.to_string(digit)