diff --git a/netutils/ptpd/ptpd.c b/netutils/ptpd/ptpd.c
index 3193ee33e..f85619093 100644
--- a/netutils/ptpd/ptpd.c
+++ b/netutils/ptpd/ptpd.c
@@ -758,11 +758,13 @@ static int ptp_sendmsg(FAR struct ptp_state_s *state, FAR const void *buf,
         0x01, 0x80, 0xc2, 0x00, 0x00, 0x0e
       };
 
-      char raw[sizeof(struct ether_header) + buflen];
+      char raw[sizeof(struct ether_header) + sizeof(struct ptp_announce_s)];
       FAR struct ether_header *header;
       struct msghdr msg;
       struct iovec iov;
 
+      DEBUGASSERT(sizeof(struct ptp_announce_s) >= buflen);
+
       header = (FAR struct ether_header *)&raw;
       memcpy(header->ether_dhost, ptp_multicast_mac, ETHER_ADDR_LEN);
       netlib_getmacaddr(state->config->interface, header->ether_shost);