diff --git a/knockout/knockout.d.ts b/knockout/knockout.d.ts
index 8b26c50b92..b8db00e3ce 100644
--- a/knockout/knockout.d.ts
+++ b/knockout/knockout.d.ts
@@ -118,7 +118,7 @@ interface KnockoutBindingContext {
     $parents: any[];
     $root: any;
     $data: any;
-    $index?: number;
+    $index?: KnockoutObservable<number>;
     $parentContext?: KnockoutBindingContext;
 
     extend(properties: any): any;