diff --git a/CSSLayout/CSSLayout.c b/CSSLayout/CSSLayout.c
index e656eeb7..958af3e2 100644
--- a/CSSLayout/CSSLayout.c
+++ b/CSSLayout/CSSLayout.c
@@ -14,7 +14,9 @@
 
 #ifdef _MSC_VER
 #include <float.h>
+#ifndef isnan
 #define isnan _isnan
+#endif
 
 #ifndef __cplusplus
 #define inline __inline