diff --git a/Makefile b/Makefile
index 7fa1e22c2..fb8fe5d41 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,9 @@
 SHELL := /bin/bash
 
+ifndef OS
+	OS := unknown
+endif
+
 ifeq ($(OS),Windows_NT)
 	ifndef ACTIVATE_SCRIPT
 		ACTIVATE_SCRIPT := pyenv/Scripts/activate
diff --git a/rspy/Makefile b/rspy/Makefile
index e961fca97..078e91386 100644
--- a/rspy/Makefile
+++ b/rspy/Makefile
@@ -1,6 +1,10 @@
 SHELL := /bin/bash
 FIND := $(if $(wildcard /bin/find),/bin/find,/usr/bin/find)
 
+ifndef OS
+	OS := unknown
+endif
+
 ifeq ($(OS),Windows_NT)
 	ifndef PYTHON_BIN
 		PYTHON_BIN := python