diff --git a/mapserver/mapfiles/overlays.map b/mapserver/mapfiles/overlays.map
index 20117becf5c247216fd7413610c78c0725df0fcc..1ccc53540e7351592f31517d233b30d60b60eef0 100644
--- a/mapserver/mapfiles/overlays.map
+++ b/mapserver/mapfiles/overlays.map
@@ -4,7 +4,7 @@ MAP
   MAXSIZE 5000
   SIZE 1024 1024
   IMAGECOLOR 128 128 128
-  IMAGETYPE rgb
+  IMAGETYPE rgba
   EXTENT -5434894.885 -5434894.885 5434894.885 5434894.885
   FONTSET "/work/mapfiles/fonts.list"