diff --git a/types/rwlock/index.d.ts b/types/rwlock/index.d.ts
index 2a034b78c3..dd7e9d2c9d 100644
--- a/types/rwlock/index.d.ts
+++ b/types/rwlock/index.d.ts
@@ -19,7 +19,7 @@ declare namespace ReadWriteLock {
   interface Options {
     scope?: any;
     timeout?: number;
-    timeoutCallback(): void;
+    timeoutCallback?(): void;
   }
 }