diff --git a/CHANGES b/CHANGES
index eb87860ab8..14f877a167 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+1842.	[port]		cmsg_len() could produce incorrect results on
+			some platform. [RT #13744]
+
 1840.	[func]		dnssec-signzone can now randomize signature end times
 			(dnssec-signzone -j jitter). [RT #13609]
 
diff --git a/lib/isc/unix/socket.c b/lib/isc/unix/socket.c
index 560115abdf..566cd9ebbf 100644
--- a/lib/isc/unix/socket.c
+++ b/lib/isc/unix/socket.c
@@ -15,7 +15,7 @@
  * PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: socket.c,v 1.237.18.10 2005/03/16 23:51:28 marka Exp $ */
+/* $Id: socket.c,v 1.237.18.11 2005/03/30 05:42:53 marka Exp $ */
 
 #include <config.h>
 
@@ -465,7 +465,11 @@ cmsg_len(ISC_SOCKADDR_LEN_T len) {
 #else
 	ISC_SOCKADDR_LEN_T hdrlen;
 
-	hdrlen = (ISC_SOCKADDR_LEN_T)CMSG_DATA(NULL); /* XXX */
+	/*
+	 * Cast NULL so that any pointer arithmetic performed by CMSG_DATA
+	 * is correct.
+	 */
+	hdrlen = (ISC_SOCKADDR_LEN_T)CMSG_DATA(((struct cmsghdr *)NULL));
 	return (hdrlen + len);
 #endif
 }