diff --git a/index.d.ts b/index.d.ts
index c92d4a4..c905f55 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -24,7 +24,7 @@ interface Opts {
   normalizePath?: NormalizePathRegexs;
   formatStatusCode?: (res: express.Response) => number | string;
 
-  transformLabels: (
+  transformLabels?: (
     labels: Labels,
     req: express.Request,
     res: express.Response