diff --git a/src/core/ngx_hash.c b/src/core/ngx_hash.c
index 0a1d69d..1f677c9 100644
--- a/src/core/ngx_hash.c
+++ b/src/core/ngx_hash.c
@@ -753,7 +753,9 @@
         k = 0;
 
         for (i = 0; i < key->len; i++) {
-            key->data[i] = ngx_tolower(key->data[i]);
+            if (!(flags & NGX_HASH_READONLY_KEY)) {
+                key->data[i] = ngx_tolower(key->data[i]);
+            }
             k = ngx_hash(k, key->data[i]);
         }