diff --git a/setup.py b/setup.py
index affd8cf..6be67e7 100644
--- a/setup.py
+++ b/setup.py
@@ -10,7 +10,7 @@
 from sys import platform
 
 if platform == 'win32':
-  environ['CFLAGS'] = '-std=c99 -D_GNU_SOURCE -_WIN32'
+  environ['CFLAGS'] = '-std=c99 -D_GNU_SOURCE -D_WIN32'
 elif platform == 'darwin':
     environ['CFLAGS'] = '-std=c99 -D_GNU_SOURCE -fcommon'
 else: