diff --git a/typescript/router/router.ts b/typescript/router/router.ts
index 857616dd6..dea087262 100644
--- a/typescript/router/router.ts
+++ b/typescript/router/router.ts
@@ -20,7 +20,7 @@ export declare class History {
 
     getFragment(fragment?: string): string;
 
-    start(op: {[key: string]: any}): void;
+    start(op?: {[key: string]: any}): void;
 
     stop(): void;