diff --git a/subprocess.hpp b/subprocess.hpp
index 8c74c2b..c8d113d 100755
--- a/subprocess.hpp
+++ b/subprocess.hpp
@@ -150,7 +150,7 @@ class OSError: public std::runtime_error
 //--------------------------------------------------------------------
 
 //Environment Variable types
-#ifndef _MSC_VER
+#ifndef __USING_WINDOWS__
 	using env_string_t = std::string;
 	using env_char_t = char;
 #else