diff --git a/m4/ax_socklen_t.m4 b/m4/ax_socklen_t.m4
index cd7cad8a..b420a170 100644
--- a/m4/ax_socklen_t.m4
+++ b/m4/ax_socklen_t.m4
@@ -55,7 +55,7 @@ getpeername(0,0,&len);
 		],
 		[[
 #include <sys/types.h>
-#ifdef WIN32
+#ifdef _WIN32
 #include <ws2tcpip.h>
 #else
 #include <sys/socket.h>