diff --git a/sys/include/net/nanocoap.h b/sys/include/net/nanocoap.h
index 720e49d6a1571..de156d59279a3 100644
--- a/sys/include/net/nanocoap.h
+++ b/sys/include/net/nanocoap.h
@@ -232,9 +232,9 @@ extern "C" {
  * @{
  */
 /** @brief    no special handling required */
-#define COAP_OPT_FINISH_NONE     (0x0)
+#define COAP_OPT_FINISH_NONE     (0x0000)
 /** @brief    expect a payload to follow */
-#define COAP_OPT_FINISH_PAYLOAD  (0x1)
+#define COAP_OPT_FINISH_PAYLOAD  (0x0001)
 /** @} */
 
 /**
@@ -542,6 +542,7 @@ size_t coap_put_block1_ok(uint8_t *pkt_pos, coap_block1_t *block1, uint16_t last
  * @param[in]     separator   character used in @p string to separate parts
  *
  * @return        number of bytes written to buffer
+ * @return        -ENOSPC if no available options
  */
 ssize_t coap_opt_add_string(coap_pkt_t *pkt, uint16_t optnum, const char *string, char separator);
 
@@ -556,6 +557,7 @@ ssize_t coap_opt_add_string(coap_pkt_t *pkt, uint16_t optnum, const char *string
  * @param[in]     value       uint to encode
  *
  * @return        number of bytes written to buffer
+ * @return        <0 reserved for error but not implemented yet
  */
 ssize_t coap_opt_add_uint(coap_pkt_t *pkt, uint16_t optnum, uint32_t value);
 
diff --git a/sys/net/application_layer/nanocoap/nanocoap.c b/sys/net/application_layer/nanocoap/nanocoap.c
index e44fd5638f3c5..d5b9c0c721877 100644
--- a/sys/net/application_layer/nanocoap/nanocoap.c
+++ b/sys/net/application_layer/nanocoap/nanocoap.c
@@ -651,6 +651,9 @@ ssize_t coap_opt_add_string(coap_pkt_t *pkt, uint16_t optnum, const char *string
         part_len = (uint8_t *)uripos - part_start;
 
         if (part_len) {
+            if (pkt->options_len == NANOCOAP_NOPTS_MAX) {
+                return -ENOSPC;
+            }
             write_len += _add_opt_pkt(pkt, optnum, part_start, part_len);
         }
     }