diff --git a/knockout/knockout.d.ts b/knockout/knockout.d.ts
index db8ab16aba..26c2821689 100644
--- a/knockout/knockout.d.ts
+++ b/knockout/knockout.d.ts
@@ -133,8 +133,8 @@ interface KnockoutAllBindingsAccessor {
 }
 
 interface KnockoutBindingHandler {
-    init?: (element: any, valueAccessor: () => any, allBindingsAccessor: KnockoutAllBindingsAccessor, viewModel: any, bindingContext: KnockoutBindingContext) => void | { controlsDescendantBindings: boolean; };
-    update?: (element: any, valueAccessor: () => any, allBindingsAccessor: KnockoutAllBindingsAccessor, viewModel: any, bindingContext: KnockoutBindingContext) => void;
+    init?: (element: any, valueAccessor: () => any, allBindingsAccessor?: KnockoutAllBindingsAccessor, viewModel?: any, bindingContext?: KnockoutBindingContext) => void | { controlsDescendantBindings: boolean; };
+    update?: (element: any, valueAccessor: () => any, allBindingsAccessor?: KnockoutAllBindingsAccessor, viewModel?: any, bindingContext?: KnockoutBindingContext) => void;
     options?: any;
     preprocess?: (value: string, name: string, addBindingCallback?: (name: string, value: string) => void) => string;
 }
@@ -641,4 +641,4 @@ declare var ko: KnockoutStatic;
 
 declare module "knockout" {
 	export = ko;
-}
\ No newline at end of file
+}