diff --git a/lib/isc/include/isc/sockaddr.h b/lib/isc/include/isc/sockaddr.h
index 3a9615c161..70ef21cd5d 100644
--- a/lib/isc/include/isc/sockaddr.h
+++ b/lib/isc/include/isc/sockaddr.h
@@ -76,11 +76,12 @@ isc_sockaddr_totext(const isc_sockaddr_t *sockaddr, isc_buffer_t *target);
 /*
  * Append a text representation of 'sockaddr' to the buffer 'target'.
  * The text will include both the IP address (v4 or v6) and the port.
- * The text is not null terminated.
+ * The text is null terminated, but the terminating null is not
+ * part of the buffer's used region.
  *
  * Returns:
  *	ISC_R_SUCCESS
- *	ISC_R_NOSPACE	The text did not fit.
+ *	ISC_R_NOSPACE	The text or the null termination did not fit.
  */
 
 ISC_LANG_ENDDECLS
diff --git a/lib/isc/sockaddr.c b/lib/isc/sockaddr.c
index 1ac08dbfb6..53ed193a0a 100644
--- a/lib/isc/sockaddr.c
+++ b/lib/isc/sockaddr.c
@@ -135,13 +135,18 @@ isc_sockaddr_totext(const isc_sockaddr_t *sockaddr, isc_buffer_t *target) {
 	plen = strlen(pbuf);
 
 	isc_buffer_available(target, &avail);
-	if (alen + 1 + plen < avail.length)
+	if (alen + 1 + plen + 1 < avail.length)
 		return (ISC_R_NOSPACE);
 	    
 	isc_buffer_putmem(target, abuf, alen);
 	isc_buffer_putmem(target, "#", 1);
 	isc_buffer_putmem(target, pbuf, plen);
 
+	/* Null terminate after used region. */
+	isc_buffer_available(target, &avail);
+	INSIST(avail.length >= 1);
+	avail.base[0] = '\0';
+
 	return (ISC_R_SUCCESS);
 }