diff --git a/types/watchpack/index.d.ts b/types/watchpack/index.d.ts
index 880e0d8a6d..4ef4ad2e67 100644
--- a/types/watchpack/index.d.ts
+++ b/types/watchpack/index.d.ts
@@ -23,7 +23,7 @@ declare class Watchpack extends EventEmitter {
 
     constructor(options: Watchpack.WatchOptions);
 
-    watch(files: string[], directories: string[], startTime: number): void;
+    watch(files: string[], directories: string[], startTime?: number): void;
 
     close(): void;
 
diff --git a/types/watchpack/watchpack-tests.ts b/types/watchpack/watchpack-tests.ts
index 7ae28bc514..f3eb459aef 100644
--- a/types/watchpack/watchpack-tests.ts
+++ b/types/watchpack/watchpack-tests.ts
@@ -2,6 +2,7 @@ import Watchpack = require('watchpack');
 const watch = new Watchpack({});
 
 watch.watch(['test.js'], ['lib/'], 1000);
+watch.watch(['test.js'], ['lib/']);
 
 let time: number;
 time = watch.getTimes()['test.js'];