diff --git a/natpmp.c b/natpmp.c
index fdf58df..c4b7bc3 100644
--- a/natpmp.c
+++ b/natpmp.c
@@ -39,12 +39,14 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include <io.h>
-#ifndef EWOULDBLOCK
-#define EWOULDBLOCK WSAEWOULDBLOCK
+#ifdef EWOULDBLOCK
+#undef EWOULDBLOCK
 #endif
-#ifndef ECONNREFUSED
-#define ECONNREFUSED WSAECONNREFUSED
+#ifdef ECONNREFUSED
+#undef ECONNREFUSED
 #endif
+#define EWOULDBLOCK WSAEWOULDBLOCK
+#define ECONNREFUSED WSAECONNREFUSED
 #include "wingettimeofday.h"
 #define gettimeofday natpmp_gettimeofday
 #else