diff --git a/src/lib/w32/posixio.c b/src/lib/w32/posixio.c
index c833e746..c5b4bc29 100644
--- a/src/lib/w32/posixio.c
+++ b/src/lib/w32/posixio.c
@@ -50,6 +50,11 @@
 #include <share.h>
 #include <stdio.h>
 #include <stdarg.h>
+/*
+ * Need to include winsock2.h for ws2tcpip.h.
+ * Use sys/socket.h to ensure the #undef NS_ALL
+ * is not missed after including winsock2.h.
+ */
 #include "sys/socket.h"
 #include <sys/stat.h>
 #include <ws2tcpip.h>