diff --git a/windows/winmisc.c b/windows/winmisc.c
index d21e1170..cd8d74ed 100644
--- a/windows/winmisc.c
+++ b/windows/winmisc.c
@@ -5,7 +5,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include "putty.h"
+#ifndef SECURITY_WIN32
 #define SECURITY_WIN32
+#endif
 #include <security.h>
 
 OSVERSIONINFO osVersion;