diff --git a/core/dnsserver/server_https.go b/core/dnsserver/server_https.go
index cddf59890..09c7d6200 100644
--- a/core/dnsserver/server_https.go
+++ b/core/dnsserver/server_https.go
@@ -188,7 +188,7 @@ func (s *ServerHTTPS) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	age := dnsutil.MinimalTTL(dw.Msg, mt)
 
 	w.Header().Set("Content-Type", doh.MimeType)
-	w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%f", age.Seconds()))
+	w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%d", uint32(age.Seconds())))
 	w.Header().Set("Content-Length", strconv.Itoa(len(buf)))
 	w.WriteHeader(http.StatusOK)
 	s.countResponse(http.StatusOK)