diff --git a/luprex/Makefile b/luprex/Makefile
index 18960b00..c7a65bb5 100644
--- a/luprex/Makefile
+++ b/luprex/Makefile
@@ -10,6 +10,8 @@ else ifneq "" "$(VSINSTALLDIR)"
 	OS=visual
 else ifneq "" "$(findstring cmd.exe,$(COMSPEC))"
 	OS=mingw
+else
+	OS=""
 endif
 
 ifeq "$(OS)" ""