diff --git a/libguile/socket.c b/libguile/socket.c
index c0faae1aa..5b17a741a 100644
--- a/libguile/socket.c
+++ b/libguile/socket.c
@@ -64,7 +64,7 @@
 
 
 #if defined (HAVE_UNIX_DOMAIN_SOCKETS) && !defined (SUN_LEN)
-#define SUN_LEN(ptr) ((size_t) (((struct sockaddr_un *) 0)->sun_path) \
+#define SUN_LEN(ptr) (offsetof (struct sockaddr_un, sun_path) \
 		      + strlen ((ptr)->sun_path))
 #endif