diff --git a/modules/httpcache/httpcache.go b/modules/httpcache/httpcache.go
index f0caa30eb82b3..4928ae5fbf98f 100644
--- a/modules/httpcache/httpcache.go
+++ b/modules/httpcache/httpcache.go
@@ -31,7 +31,7 @@ func AddCacheControlToHeader(h http.Header, maxAge time.Duration, additionalDire
 		directives = append(directives, "max-age=0", "private", "must-revalidate")
 
 		// to remind users they are using non-prod setting.
-		h.Add("X-Gitea-Debug", "RUN_MODE="+setting.RunMode)
+		h.Set("X-Gitea-Debug", "RUN_MODE="+setting.RunMode)
 	}
 
 	h.Set("Cache-Control", strings.Join(append(directives, additionalDirectives...), ", "))