diff --git a/render/render.go b/render/render.go
index 71852364..baa8f337 100644
--- a/render/render.go
+++ b/render/render.go
@@ -28,7 +28,7 @@ var (
 
 func writeContentType(w http.ResponseWriter, value []string) {
 	header := w.Header()
-	if val := header["Content-Type"]; len(val) == 0 {
-		header["Content-Type"] = value
+	if header.Get("Content-Type") == "" {
+		header.Set("Content-Type", value)
 	}
 }