--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -1196,7 +1196,6 @@ static ngx_int_t
 ngx_http_proxy_process_header(ngx_http_request_t *r)
 {
     ngx_int_t                       rc;
-    ngx_uint_t                      i;
     ngx_table_elt_t                *h;
     ngx_http_upstream_header_t     *hh;
     ngx_http_upstream_main_conf_t  *umcf;
@@ -1237,9 +1236,7 @@ ngx_http_proxy_process_header(ngx_http_r
                 ngx_memcpy(h->lowcase_key, r->lowcase_header, h->key.len);
 
             } else {
-                for (i = 0; i < h->key.len; i++) {
-                    h->lowcase_key[i] = ngx_tolower(h->key.data[i]);
-                }
+                ngx_strlow(h->lowcase_key, h->key.data, h->key.len);
             }
 
             hh = ngx_hash_find(&umcf->headers_in_hash, h->hash,