diff --git a/src/core/ddsi/include/dds/ddsi/q_protocol.h b/src/core/ddsi/include/dds/ddsi/q_protocol.h
index 9d5495c..5a5d6e0 100644
--- a/src/core/ddsi/include/dds/ddsi/q_protocol.h
+++ b/src/core/ddsi/include/dds/ddsi/q_protocol.h
@@ -232,7 +232,7 @@ typedef struct AckNack {
   /* nn_count_t count; */
 } AckNack_t;
 #define ACKNACK_FLAG_FINAL 0x02u
-#define ACKNACK_SIZE(numbits) (offsetof (AckNack_t, bits) + NN_SEQUENCE_NUMBER_SET_SIZE (numbits) + 4)
+#define ACKNACK_SIZE(numbits) (offsetof (AckNack_t, bits) + NN_SEQUENCE_NUMBER_SET_BITS_SIZE (numbits) + 4)
 #define ACKNACK_SIZE_MAX ACKNACK_SIZE (256u)
 
 typedef struct Gap {
@@ -243,7 +243,7 @@ typedef struct Gap {
   nn_sequence_number_set_header_t gapList;
   uint32_t bits[];
 } Gap_t;
-#define GAP_SIZE(numbits) (offsetof (Gap_t, bits) + NN_SEQUENCE_NUMBER_SET_SIZE (numbits))
+#define GAP_SIZE(numbits) (offsetof (Gap_t, bits) + NN_SEQUENCE_NUMBER_SET_BITS_SIZE (numbits))
 #define GAP_SIZE_MAX GAP_SIZE (256u)
 
 typedef struct InfoTS {
@@ -281,7 +281,7 @@ typedef struct NackFrag {
   uint32_t bits[];
   /* nn_count_t count; */
 } NackFrag_t;
-#define NACKFRAG_SIZE(numbits) (offsetof (NackFrag_t, fragmentNumberState) + NN_FRAGMENT_NUMBER_SET_SIZE (numbits) + 4)
+#define NACKFRAG_SIZE(numbits) (offsetof (NackFrag_t, bits) + NN_FRAGMENT_NUMBER_SET_BITS_SIZE (numbits) + 4)
 #define NACKFRAG_SIZE_MAX NACKFRAG_SIZE (256u)
 
 typedef struct PT_InfoContainer {
diff --git a/src/core/ddsi/src/q_receive.c b/src/core/ddsi/src/q_receive.c
index a7d4036..0477aee 100644
--- a/src/core/ddsi/src/q_receive.c
+++ b/src/core/ddsi/src/q_receive.c
@@ -133,7 +133,7 @@ static int valid_AckNack (AckNack_t *msg, size_t size, int byteswap)
      submessage, and verify that the submessage is large enough */
   if (size < ACKNACK_SIZE (msg->readerSNState.numbits))
     return 0;
-  count = (nn_count_t *) ((char *) &msg->readerSNState + NN_SEQUENCE_NUMBER_SET_SIZE (msg->readerSNState.numbits));
+  count = (nn_count_t *) ((char *) &msg->bits + NN_SEQUENCE_NUMBER_SET_BITS_SIZE (msg->readerSNState.numbits));
   if (byteswap)
   {
     bswap_sequence_number_set_bitmap (&msg->readerSNState, msg->bits);
@@ -283,8 +283,7 @@ static int valid_NackFrag (NackFrag_t *msg, size_t size, int byteswap)
      submessage, and verify that the submessage is large enough */
   if (size < NACKFRAG_SIZE (msg->fragmentNumberState.numbits))
     return 0;
-  count = (nn_count_t *) ((char *) &msg->fragmentNumberState +
-                          NN_FRAGMENT_NUMBER_SET_SIZE (msg->fragmentNumberState.numbits));
+  count = (nn_count_t *) ((char *) &msg->bits + NN_FRAGMENT_NUMBER_SET_BITS_SIZE (msg->fragmentNumberState.numbits));
   if (byteswap)
   {
     bswap_fragment_number_set_bitmap (&msg->fragmentNumberState, msg->bits);
@@ -651,8 +650,7 @@ static int handle_AckNack (struct receiver_state *rst, nn_etime_t tnow, const Ac
   struct whc_state whcst;
   int hb_sent_in_response = 0;
   memset (gapbits, 0, sizeof (gapbits));
-  countp = (nn_count_t *) ((char *) msg + offsetof (AckNack_t, readerSNState) +
-                           NN_SEQUENCE_NUMBER_SET_SIZE (msg->readerSNState.numbits));
+  countp = (nn_count_t *) ((char *) msg + offsetof (AckNack_t, bits) + NN_SEQUENCE_NUMBER_SET_BITS_SIZE (msg->readerSNState.numbits));
   src.prefix = rst->src_guid_prefix;
   src.entityid = msg->readerId;
   dst.prefix = rst->dst_guid_prefix;
@@ -1382,7 +1380,7 @@ static int handle_NackFrag (struct receiver_state *rst, nn_etime_t tnow, const N
   nn_count_t *countp;
   seqno_t seq = fromSN (msg->writerSN);
 
-  countp = (nn_count_t *) ((char *) msg + offsetof (NackFrag_t, fragmentNumberState) + NN_FRAGMENT_NUMBER_SET_SIZE (msg->fragmentNumberState.numbits));
+  countp = (nn_count_t *) ((char *) msg + offsetof (NackFrag_t, bits) + NN_FRAGMENT_NUMBER_SET_BITS_SIZE (msg->fragmentNumberState.numbits));
   src.prefix = rst->src_guid_prefix;
   src.entityid = msg->readerId;
   dst.prefix = rst->dst_guid_prefix;
diff --git a/src/core/ddsi/src/q_xevent.c b/src/core/ddsi/src/q_xevent.c
index 4588c0d..d4de160 100644
--- a/src/core/ddsi/src/q_xevent.c
+++ b/src/core/ddsi/src/q_xevent.c
@@ -699,7 +699,7 @@ static void add_AckNack (struct nn_xmsg *msg, struct proxy_writer *pwr, struct p
   seqno_t base;
 
   DDSRT_STATIC_ASSERT ((NN_FRAGMENT_NUMBER_SET_MAX_BITS % 32) == 0);
-  union {
+  struct {
     struct nn_fragment_number_set_header set;
     uint32_t bits[NN_FRAGMENT_NUMBER_SET_MAX_BITS / 32];
   } nackfrag;
@@ -788,8 +788,7 @@ static void add_AckNack (struct nn_xmsg *msg, struct proxy_writer *pwr, struct p
   {
     /* Count field is at a variable offset ... silly DDSI spec. */
     nn_count_t *countp =
-      (nn_count_t *) ((char *) an + offsetof (AckNack_t, readerSNState) +
-                      NN_SEQUENCE_NUMBER_SET_SIZE (an->readerSNState.numbits));
+      (nn_count_t *) ((char *) an + offsetof (AckNack_t, bits) + NN_SEQUENCE_NUMBER_SET_BITS_SIZE (an->readerSNState.numbits));
     *countp = ++rwn->count;
 
     /* Reset submessage size, now that we know the real size, and update
@@ -824,7 +823,7 @@ static void add_AckNack (struct nn_xmsg *msg, struct proxy_writer *pwr, struct p
 
     {
       nn_count_t *countp =
-        (nn_count_t *) ((char *) nf + offsetof (NackFrag_t, fragmentNumberState) + NN_FRAGMENT_NUMBER_SET_SIZE (nf->fragmentNumberState.numbits));
+        (nn_count_t *) ((char *) nf + offsetof (NackFrag_t, bits) + NN_FRAGMENT_NUMBER_SET_BITS_SIZE (nf->fragmentNumberState.numbits));
       *countp = ++pwr->nackfragcount;
       nn_xmsg_submsg_setnext (msg, sm_marker);