diff --git a/sys/include/net/gnrc/udp.h b/sys/include/net/gnrc/udp.h
index 91e4bbe9818d..2b19e9887629 100644
--- a/sys/include/net/gnrc/udp.h
+++ b/sys/include/net/gnrc/udp.h
@@ -76,6 +76,8 @@ int gnrc_udp_calc_csum(gnrc_pktsnip_t *hdr, gnrc_pktsnip_t *pseudo_hdr);
  * @return  pointer to the newly created (and allocated) header
  * @return  NULL on `src == NULL`, `dst == NULL`, `src_len != 2`, `dst_len != 2`
  *          or on allocation error
+ *
+ * @pre     `src > 0` and `dst > 0`
  */
 gnrc_pktsnip_t *gnrc_udp_hdr_build(gnrc_pktsnip_t *payload, uint16_t src,
                                    uint16_t dst);
diff --git a/sys/net/gnrc/transport_layer/udp/gnrc_udp.c b/sys/net/gnrc/transport_layer/udp/gnrc_udp.c
index 2bc293f4b46f..ceeb4d33a09a 100644
--- a/sys/net/gnrc/transport_layer/udp/gnrc_udp.c
+++ b/sys/net/gnrc/transport_layer/udp/gnrc_udp.c
@@ -280,6 +280,8 @@ int gnrc_udp_calc_csum(gnrc_pktsnip_t *hdr, gnrc_pktsnip_t *pseudo_hdr)
 gnrc_pktsnip_t *gnrc_udp_hdr_build(gnrc_pktsnip_t *payload, uint16_t src,
                                    uint16_t dst)
 {
+    assert((src > 0) && (dst > 0));
+
     gnrc_pktsnip_t *res;
     udp_hdr_t *hdr;