diff --git a/sys/include/net/nanocoap.h b/sys/include/net/nanocoap.h
index 4d9a916056..083815af59 100644
--- a/sys/include/net/nanocoap.h
+++ b/sys/include/net/nanocoap.h
@@ -1269,12 +1269,32 @@ static inline size_t coap_opt_put_block2_control(uint8_t *buf, uint16_t lastonum
  *                          or 0 if first option
  * @param[in]   optnum      option number to use
  * @param[in]   string      string to encode as option
+ * @param[in]   len         length of the string
  * @param[in]   separator   character used in @p string to separate parts
  *
  * @return      number of bytes written to @p buf
  */
-size_t coap_opt_put_string(uint8_t *buf, uint16_t lastonum, uint16_t optnum,
-                           const char *string, char separator);
+size_t coap_opt_put_string_with_len(uint8_t *buf, uint16_t lastonum, uint16_t optnum,
+                                    const char *string, size_t len, char separator);
+/**
+ * @brief   Encode the given string as multi-part option into buffer
+ *
+ * @param[out]  buf         buffer to write to
+ * @param[in]   lastonum    number of previous option (for delta calculation),
+ *                          or 0 if first option
+ * @param[in]   optnum      option number to use
+ * @param[in]   string      string to encode as option
+ * @param[in]   separator   character used in @p string to separate parts
+ *
+ * @return      number of bytes written to @p buf
+ */
+static inline size_t coap_opt_put_string(uint8_t *buf, uint16_t lastonum,
+                                         uint16_t optnum,
+                                         const char *string, char separator)
+{
+    return coap_opt_put_string_with_len(buf, lastonum, optnum,
+                                        string, strlen(string), separator);
+}
 
 /**
  * @brief   Convenience function for inserting LOCATION_PATH option into buffer
diff --git a/sys/net/application_layer/nanocoap/nanocoap.c b/sys/net/application_layer/nanocoap/nanocoap.c
index 8d62d987cc..bdb4a6bfd3 100644
--- a/sys/net/application_layer/nanocoap/nanocoap.c
+++ b/sys/net/application_layer/nanocoap/nanocoap.c
@@ -747,11 +747,9 @@ size_t coap_opt_put_block(uint8_t *buf, uint16_t lastonum, coap_block_slicer_t *
     return coap_opt_put_uint(buf, lastonum, option, _slicer2blkopt(slicer, more));
 }
 
-size_t coap_opt_put_string(uint8_t *buf, uint16_t lastonum, uint16_t optnum,
-                           const char *string, char separator)
+size_t coap_opt_put_string_with_len(uint8_t *buf, uint16_t lastonum, uint16_t optnum,
+                                    const char *string, size_t len, char separator)
 {
-    size_t len = strlen(string);
-
     if (len == 0) {
         return 0;
     }