diff --git a/render/render.go b/render/render.go
index bc7bceb8..95c0a93a 100644
--- a/render/render.go
+++ b/render/render.go
@@ -54,7 +54,9 @@ var (
 )
 
 func writeHeader(w http.ResponseWriter, code int, contentType string) {
-	w.Header().Set("Content-Type", contentType+"; charset=utf-8")
+	if w.Header().Get("Content-Type") == "" {
+		w.Header().Set("Content-Type", contentType+"; charset=utf-8")
+	}
 	w.WriteHeader(code)
 }