diff --git a/Makefile b/Makefile
index 7915402e..083f3bd8 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-ifeq ($(OS),Windows_NT)
+ifeq ($(shell uname -o),Cygwin)
 WUT_ROOT := $(shell cygpath -w ${CURDIR})
 else
 WUT_ROOT := $(CURDIR)
diff --git a/rpl/common/rules.mk b/rpl/common/rules.mk
index bce0c4b0..0a63e180 100644
--- a/rpl/common/rules.mk
+++ b/rpl/common/rules.mk
@@ -1,6 +1,6 @@
 .SUFFIXES:
 
-ifeq ($(OS),Windows_NT)
+ifeq ($(shell uname -o),Cygwin)
 CUR_DIR := $(shell cygpath -w ${CURDIR})
 else
 CUR_DIR := $(CURDIR)
diff --git a/samples/helloworld/Makefile b/samples/helloworld/Makefile
index 9946da1f..2db617e0 100644
--- a/samples/helloworld/Makefile
+++ b/samples/helloworld/Makefile
@@ -4,7 +4,7 @@ ifeq ($(strip $(WUT_ROOT)),)
 $(error "Please ensure WUT_ROOT is in your environment.")
 endif
 
-ifeq ($(OS),Windows_NT)
+ifeq ($(shell uname -o),Cygwin)
 ROOT := $(shell cygpath -w ${CURDIR})
 WUT_ROOT := $(shell cygpath -w ${WUT_ROOT})
 else