diff --git a/pkg/tinydtls/contrib/sock_dtls.c b/pkg/tinydtls/contrib/sock_dtls.c
index 3692f36a13..315d80d076 100644
--- a/pkg/tinydtls/contrib/sock_dtls.c
+++ b/pkg/tinydtls/contrib/sock_dtls.c
@@ -515,7 +515,6 @@ ssize_t sock_dtls_recv_aux(sock_dtls_t *sock, sock_dtls_session_t *remote,
                            void *data, size_t max_len, uint32_t timeout,
                            sock_dtls_aux_rx_t *aux)
 {
-    (void)aux;
     assert(sock);
     assert(data);
     assert(remote);
@@ -533,8 +532,13 @@ ssize_t sock_dtls_recv_aux(sock_dtls_t *sock, sock_dtls_session_t *remote,
                  msg.type == DTLS_EVENT_CONNECTED) {
             return _complete_handshake(sock, remote, msg.content.ptr);
         }
-        res = sock_udp_recv(sock->udp_sock, data, max_len, timeout,
-                            &remote->ep);
+        /* Crude way to somewhat test that `sock_dtls_aux_rx_t` and
+         * `sock_udp_aux_rx_t` remain compatible: */
+        static_assert(sizeof(sock_dtls_aux_rx_t) == sizeof(sock_udp_aux_rx_t),
+                      "sock_dtls_aux_rx_t became incompatible with "
+                      "sock_udp_aux_rx_t");
+        res = sock_udp_recv_aux(sock->udp_sock, data, max_len, timeout,
+                                &remote->ep, (sock_udp_aux_rx_t *)aux);
         if (res <= 0) {
             DEBUG("sock_dtls: error receiving UDP packet: %d\n", (int)res);
             return res;