diff --git a/gulp-mocha/gulp-mocha.d.ts b/gulp-mocha/gulp-mocha.d.ts
index e6bc099c3..8d21b1323 100644
--- a/gulp-mocha/gulp-mocha.d.ts
+++ b/gulp-mocha/gulp-mocha.d.ts
@@ -7,6 +7,6 @@
 /// <reference path="../node/node.d.ts"/>
 
 declare module "gulp-mocha" {
-    function mocha(setupOptions: MochaSetupOptions): NodeJS.ReadWriteStream;
+    function mocha(setupOptions?: MochaSetupOptions): NodeJS.ReadWriteStream;
     export = mocha;
 }
\ No newline at end of file