diff --git a/jquery/jquery.d.ts b/jquery/jquery.d.ts
index ad60f9670..59159a188 100644
--- a/jquery/jquery.d.ts
+++ b/jquery/jquery.d.ts
@@ -176,7 +176,7 @@ interface JQueryXHR extends XMLHttpRequest, JQueryPromise<any> {
     /**
      * Incorporates the functionality of the .done() and .fail() methods, allowing (as of jQuery 1.8) the underlying Promise to be manipulated. Refer to deferred.then() for implementation details.
      */
-    then(doneCallback: (data: any, textStatus: string, jqXHR: JQueryXHR) => void, failCallback: (jqXHR: JQueryXHR, textStatus: string, errorThrown: any) => void): JQueryPromise<any>;
+    then(doneCallback: (data: any, textStatus: string, jqXHR: JQueryXHR) => void, failCallback?: (jqXHR: JQueryXHR, textStatus: string, errorThrown: any) => void): JQueryPromise<any>;
     /**
      * Property containing the parsed response if the response Content-Type is json
      */