pass length clearly
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 57f4083..0956f01 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2636,7 +2636,7 @@
 
     if (r->upstream->rewrite_redirect) {
 
-        p = ngx_strcasestrn(ho->value.data, "url=", 3);
+        p = ngx_strcasestrn(ho->value.data, "url=", 4 - 1);
 
         if (p) {
             rc = r->upstream->rewrite_redirect(r, ho, p + 4 - ho->value.data);