diff -Naur ocaml-3.12.1.orig/otherlibs/win32unix/socket.c ocaml-3.12.1/otherlibs/win32unix/socket.c
--- ocaml-3.12.1.orig/otherlibs/win32unix/socket.c	2010-01-22 12:48:24.000000000 +0000
+++ ocaml-3.12.1/otherlibs/win32unix/socket.c	2011-11-14 12:49:18.000000000 +0000
@@ -17,7 +17,7 @@
 #include "unixsupport.h"

 int socket_domain_table[] = {
-  PF_UNIX, PF_INET
+  PF_UNIX, PF_INET /*, PF_INET6 */
 };

 int socket_type_table[] = {
@@ -30,6 +30,12 @@
   SOCKET s;
   int oldvalue, oldvaluelen, newvalue, retcode;

+  /* IPv6 requires WinSock2, we must raise an error on PF_INET6 */
+  if (Int_val(domain) >= sizeof(socket_domain_table)/sizeof(int)) {
+    win32_maperr(WSAEPFNOSUPPORT);
+    uerror("socket", Nothing);
+  }
+
   oldvaluelen = sizeof(oldvalue);
   retcode = getsockopt(INVALID_SOCKET, SOL_SOCKET, SO_OPENTYPE,
                        (char *) &oldvalue, &oldvaluelen);