diff --git a/sys/include/net/nanocoap.h b/sys/include/net/nanocoap.h
index 035db1782a..8da3b9e75c 100644
--- a/sys/include/net/nanocoap.h
+++ b/sys/include/net/nanocoap.h
@@ -1763,6 +1763,7 @@ static inline size_t coap_opt_put_uri_query(uint8_t *buf, uint16_t lastonum,
  * @param[out]  buf         buffer to write to
  * @param[in,out] lastonum  number of previous option (for delta calculation),
  *                          or 0 if first option
+ *                          May be NULL, then previous option is assumed to be 0.
  * @param[in]   uri         ptr into a source URI, to the first character after
  *                          the authority component
  *
diff --git a/sys/net/application_layer/nanocoap/nanocoap.c b/sys/net/application_layer/nanocoap/nanocoap.c
index a058ae1232..ad5311507c 100644
--- a/sys/net/application_layer/nanocoap/nanocoap.c
+++ b/sys/net/application_layer/nanocoap/nanocoap.c
@@ -904,6 +904,7 @@ size_t coap_opt_put_uri_pathquery(uint8_t *buf, uint16_t *lastonum, const char *
 {
     size_t len;
     const char *query = strchr(uri, '?');
+    uint16_t _lastonum = lastonum ? *lastonum : 0;
 
     if (query) {
         len = (query == uri) ? 0 : (query - uri - 1);
@@ -911,16 +912,20 @@ size_t coap_opt_put_uri_pathquery(uint8_t *buf, uint16_t *lastonum, const char *
         len = strlen(uri);
     }
 
-    size_t bytes_out = coap_opt_put_string_with_len(buf, *lastonum,
+    size_t bytes_out = coap_opt_put_string_with_len(buf, _lastonum,
                                                     COAP_OPT_URI_PATH,
                                                     uri, len, '/');
     if (query) {
         buf += bytes_out;
         bytes_out += coap_opt_put_uri_query(buf, COAP_OPT_URI_PATH, query + 1);
-        *lastonum = COAP_OPT_URI_QUERY;
+        _lastonum = COAP_OPT_URI_QUERY;
     }
     else if (bytes_out) {
-        *lastonum = COAP_OPT_URI_PATH;
+        _lastonum = COAP_OPT_URI_PATH;
+    }
+
+    if (lastonum) {
+        *lastonum = _lastonum;
     }
 
     return bytes_out;