diff --git a/src/ndisc/nm-lndp-ndisc.c b/src/ndisc/nm-lndp-ndisc.c
index 22695c32db..c302f3b2c1 100644
--- a/src/ndisc/nm-lndp-ndisc.c
+++ b/src/ndisc/nm-lndp-ndisc.c
@@ -333,7 +333,12 @@ _ndp_msg_add_option(struct ndp_msg *msg, int len)
     return ret;
 }
 
+/*****************************************************************************/
+
+/* "Recursive DNS Server Option" at https://tools.ietf.org/html/rfc8106#section-5.1 */
+
 #define NM_ND_OPT_RDNSS 25
+
 typedef struct {
     struct nd_opt_hdr header;
     uint16_t          reserved;
@@ -341,7 +346,14 @@ typedef struct {
     struct in6_addr   addrs[0];
 } NMLndpRdnssOption;
 
+G_STATIC_ASSERT(sizeof(NMLndpRdnssOption) == 8u);
+
+/*****************************************************************************/
+
+/* "DNS Search List Option" at https://tools.ietf.org/html/rfc8106#section-5.2 */
+
 #define NM_ND_OPT_DNSSL 31
+
 typedef struct {
     struct nd_opt_hdr header;
     uint16_t          reserved;
@@ -349,6 +361,10 @@ typedef struct {
     char              search_list[0];
 } NMLndpDnsslOption;
 
+G_STATIC_ASSERT(sizeof(NMLndpDnsslOption) == 8u);
+
+/*****************************************************************************/
+
 static gboolean
 send_ra(NMNDisc *ndisc, GError **error)
 {