diff --git a/Arbiter/Arbiter.d.ts b/Arbiter/Arbiter.d.ts
index ebe2d1656..d80661e58 100644
--- a/Arbiter/Arbiter.d.ts
+++ b/Arbiter/Arbiter.d.ts
@@ -7,7 +7,7 @@
 
 declare module ArbiterDef {
     export interface SubscribeHandler {
-        (data?: any, message?: string, subscriber_context?: any): void;
+        (data: any, message: string, subscriber_context: any): void;
     }
 
     export interface SubscribeOptions {