diff -up mtr-0.75/net.c.fd-flags mtr-0.75/net.c
--- mtr-0.75/net.c.fd-flags	2008-10-22 08:07:53.000000000 +0200
+++ mtr-0.75/net.c	2008-10-23 13:57:04.000000000 +0200
@@ -945,8 +945,9 @@ int net_preopen(void) 
   set_fd_flags(recvsock4);
 #ifdef ENABLE_IPV6
   recvsock6 = socket(AF_INET6, SOCK_RAW, IPPROTO_ICMPV6);
+  if (recvsock6 >= 0)
+    set_fd_flags(recvsock6);  
 #endif
-  set_fd_flags(recvsock6);
 
   return 0;
 }